Simply what the subject says.
You're walking along a clockworks level and you find recon modules that expire in 24 hours from everyone who's been there and bothered to care writing one. But you can't read it until all monsters in the room are cleared.
This recon module says, "To all knights, the last wave is all Greavers. GL!!!"
This recon module says, "This Danger Room has fiends only. NOT WORTH IT!"
This recon module says, "Jimmy died here. :P"
This recon module says, "East path: Ele, south path: Loot"
This recon module says, "WTS asi h brandish!! PM BestSuggestionEver!"
Recon modules should be prize wheel (in sums of 3) or low price like 200 cr. Or if you use both of your lives, you should get a free recon module. Guild halls and haven would be cool too. But not in normal missions.
To prevent spam, set a limit to the amount you can place, like only 1 module per depth. One problem, if you can't read them until the monsters are clear what is the point? Aren't they supposed to be their to inform/help you? Other than that it is a good idea.