It's almost always bordered on three sides. If you have a bomb or two, it's worth looking for the secret room.When an item says it 'appeared in the basement' that just means it will now spawn (some items you have to do particular things before they'll spawn) it doesn't necessarily mean that they're somewhere in your current game.There are some rocks that are suspiciously dark-grey.
There's a pattern to where it spawns, though - it can only spawn in a 'blank' map tile with at least 3 rooms touching it (except occasionally on the first floor where there sometimes are no such groupings of rooms) A couple items will tell you where it is and you can find it by bombing walls. Always have at least one key to open that room.
There's an item on every floor in a special room marked by a crown on the map and a yellow border around the door, but on every floor besides the first it's locked.
