It's hard to describe exactly where they are but on level 3/5 of Operation Crimson Hammer when you get to the part where there are loads of tough blocks (3 hit's to break) and you need 4 buttons to open the gate, at that point Gremlin Bombs will follow you where ever you go until you get to the top where there are Green Treasure Boxes. There are also the stealth Gremlins. Well at that point if you go right at the bottom of that part, you cannot see it however if you spam your Handgun South it should hit a Ghost Block, opening up a little path where more Bombs follow you and there are 2 (I think) rare treasure Boxes.

I don't know if people know about this already but I discovered it a couple of months ago :P
Apologies if you already knew about it.

~The-Mighty-Potato