This is a guide on how to find and unlock secret rooms!
Secret Rooms, How and Where to Find Them
As pictured here:
Or a sneakier one here:
These (to my knowledge), can appear in any normal fight room on the floor. This means event, blacksmith, tavern, shop, boss, and hatch rooms can’t have secret rooms in them.
Oh, and for extra usefulness, go ahead and try to hit a few enemies with that bomb you placed to open the room.