Little Dungeon Secrets?
I've found one, so far. I don't know what the floor was called, but it's the one with a lot of grass everywhere and boxes floating off to the sides. Shooting the boxes, I found a switch which spawned a key and a few enemies. The key opened a door to some treasure that would normally be missed.
I've also noticed a weird little arrow symbol made out of breakable grass blocks in some levels, and wondered if it led to anything. If it does, I haven't been able to figure it out.
Are there any other secrets like this?

not behind the door, but behind the ROOM, always look behind the walls of those rooms (usually the access its OUTSIDE the room)
something like this
...| |............... _______
...| |................|...........|
...| |................|...........|
__| |_________|..........|
..........=====_______|
..........|.........|
..........|_....__|
.............| |
Ignore the dots, thats empty space, EQUALS its the secret passageway.
LOL, not sure if it makes sense, but the upper room its the secret one, and the lower room its the treasure box room.
I KNEW those floating rocks had a switch!
I've been breaking the things every time I've been to the floor though, and nothing x.x
Oh, and btw, the little red pressure valves? Not always useless.
I tried that philosophy IRL once, Skype.
Didn't work out so well.
In the Scarlet Fortress there's a level where you can remove a wall only if you do it before stepping on a party button. You need to shoot it / throw a vase at it from a different room, across a chasm. Once you step on the party button in the corridor, you won't be able to do that anymore. It gives access to a little extra loot.

i haven't seen anything beyond the three or so mentioned here
Every graveyard level has one variant of secret passage or other. Literally, EVERY graveyard. Keep an eye out for two loose treasure boxes just sitting on an "Unreachable" platform, there's actually a small pathway that leads to em from behind a small tomb that obscures the passage from the camera's perspective. See N01r's post for details.

In one of the common sunshiney grass levels (generally tier-1...is it Aurora Isles? or maybe a Wolver Den?) the level ends with a team-switch arena room that has three waves of enemies (generally just wolvers and wings). The entrance is in the northeast and the way to the lift opens in the west after the battle. But THERE IS A DOOR TO THE SOUTH. How do you open it? Last time I soloed that level I destroyed everything within the room to check I wasn't missing an obvious switch, so I suspect it must be something earlier. Anyone know this one? This isn't the floating box level the OP was talking about, is it? I am blind to background items, like floating boxes.
@Ecliptic
You need to enter that section BEFORE getting to the arena-room you mention. Enter from the room south-east of the arena, it's hidden behind a whole bunch of breakable blocks.

We love these little bonus secrets that you learn over time and it's something we'll keep adding here and there as we add new content to the game.
When I played games growing up, secrets in games were everything. It's hard to emulate that these days though. Thanks for nothing Internet.
that's why I'm a little leery of the wiki. Especially this new initiative to map every level of the clockworks.
I don't wanna be able to just look up whatever I want at a moments notice...
I liked finding the Cane of Byrna on my own, thankyouverymuch.

I want to be able to push aside a tomb in a graveyard and go down into an area full of re-zombies.
Wait, I actually do not want that at all.

@Nick is there a way to make this secrets totally random? maybe even make us START activating certain switches on the first levels and combine with other switches to activate in lower levels so as to open a secret passageway on an even later level?
thats a good idea. maybe have 1 secret switch in each level and if you find every secret switch in a tier the last level has a room with a 25% chance that you'll get a rare recipe. if it takes more than 15 minutes to find all the switches it wont end up on youtube and everybody would be too lazy to tell about it
I still want to know what the switch at the beginning of the last stage of citadel before Vanaduke does.
It's hidden under a crate with fire around it.
that's why I'm a little leery of the wiki. Especially this new initiative to map every level of the clockworks.
I don't wanna be able to just look up whatever I want at a moments notice...
I liked finding the Cane of Byrna on my own, thankyouverymuch.
Epic Zelda Reference for teh win!
LOZ LTTP is probably the second best zelda game ever made.

I lol'd about Nick's post. Least it wasn't me *commits other's notes to memory*
GodofSkype1: "LOZ LTTP is probably the second best zelda game ever made."
Only second? Please stop trolling.

"probably" ensures that it wasn't a definitely factual statement, it could or could not be since it was "probably". It's an opinion. Don't make it a problem. Anyway, that's like arguing that the classic Sonic is better than Modern Sonic. It's not worth getting into and is definitely against the spirit of the original comment which truly was about a reference to Zelda being good.
GodofSkype1: "LOZ LTTP is probably the second best zelda game ever made."
Only second? Please stop trolling.
Yes, Legend Of Zelda: Link To The Past is the SECOND best Zelda game ever made.
Legend Of Zelda: Ocarina Of Time is the BEST
NOT TROLLING

=.= you just had too change it to "IS"...
*runs away from opinions*
"Yes, Legend Of Zelda: Link To The Past is the SECOND best Zelda game ever made.
Legend Of Zelda: Ocarina Of Time is the BEST"
MAXIMUM TROLLING!
My contribution: near the beginning of one of the really common grass levels there are two group buttons, to the left and right before a short turret gauntlet with spikes. The right-hand button is optional. When you activate it, you hear the "DUN-DUN" sound, which is BAD. That button activates the turrets, and all you get is a few crowns.
Go to the Zelda community websites and ask them because I'm telling that's majority opinion.
Starting all this Zelda talk is giving me nostalgia *ahhh*
=.= you just had too change it to "IS"...
*runs away from opinions*
LOL, I thought I had IS originally.
The Jelly Farm [1]
There are two possible arenas at the beginning. One in the basic path and another behind an energy gate path. It's possible to do one arena, and then go back and do the other one for maximum heat, crown, etc.
After you finish one [or both] of these arenas, you are led into a sort of farm area with about 5-7 rows of grass patches. Walking towards the treasure boxes in each summons one to two lumbers.
This is all normal. But in the farm like area, you can see another section of the map that's closed off. It has an arena button, and several pressure buttons [which can be seen by throwing a potion at a very precise angle towards some crystal shards, which break apart and reveal two pressure button.]
I have not yet found a way into that segment of the map. It would be nice if someone who know would tell me.
Tsuki, I would like to know that too. I've been wondering that pretty much since that level showed up during beta.
I eventually gave up and figured it was left over after the level was edited, and they kept it just to tease us!

It was an old danger room, that had special green jellies and blast cubes in it.
I think they should bring it back.
The room Ecliptic was asking about is the same one I was asking about. It's in the same stage as the weird arrow design. I still don't understand how to get into that door.

There's a very nice secret on the floor just before Vanaduke. To hide it I've coded it, if you want the secret just flow this rule:
Every 2 letters/numbers.
SHbist ia tswwtigthcdh gatt ltyhse ysntaafrgt jouf jtuhse jleemvreil ftko mtsudrjn ludnrbsrsekackdafbtlde dbfrhiscdkss tagt ftjhse dehndd gtso gmsavgjicc fbarjisclkis gtso cfhiancd 42 ftsrjecayscufrae dbjosxheas.
You want the spoilers, work it out >:)

Everyone knows about the FSC button. I'm not even bother decoding it.
Great job necroing a thread from a year ago.
I've got a better one. At the... third floor of OCH, just after the part where you navigate a series of fallen girders, you end up at a place with stone blocks, a straight path, and ninja-recons. There are some brown blocks to your lower-right. Fire a gun at their general direction and you should hit a ghost block that leads to three red chests. \o/

So has someone figured out how to get those heart boxes in Heart of Ice? Oh, how they taunt me...

there are two locations for the switch which is hidden in a stone block:
1) in the block right below the button (This one is obvious)
2) on the left where you would need to hit it with a gun or projectile.
The switch spawns a ghost block to get to the heart boxes.
Edit:
I should be clear, this is before you step onto the 2nd button of that level (the first being to enter the stage.) In the area where you need to put all the statues on the pressure plates.

Hit a switch at the start of the level to turn unbreakable bricks at the end to magic bricks to find 2 treasure boxes.
o yjpihjy upi vpfrf oy
Go play Ocarina of time, and do the water temple (master quest). After this you will be one with hidden secrets.

Lol I got another secret.
In the boss vanaduke, if you poison him, his attacks get weaker.
So for ppl that struggle on phase 2 and 4, poison his mask when u water it.
barely takes a bar or 2 ^^

poison is already supposed to weaken you or a monsters attack

there are 2 stone blocks on the left along the wall, 1 at the pad, but then another at the first group of walled in fiends.

heres the level type you were talking about
http://wiki.spiralknights.com/Aurora_Isles
the level itself is called stone grove

Just thinking of the secrets bring my memories back of those "noob days" of Spiral Knights. I'd love to find more secrets -- there's sure to be more! :D

Who would want to start another secrets thread??? I sure don't. /shrug

Maybe there are, you're just not looking hard enough. ;D

Maybe this should be "Necro the Secrets Thread" Day.
The 1st International Forum Holiday.
I cant believe no one followed through with the tradition
While running fearing for your life and sanity in graveyards keep an eye out. Some of the rooms with an energy door have a secret passage behind the door that leads to a few treasure boxes. It is hard to see on the screen, but the minimap gives it away as you can see a smallish square jutting out behind energy room.