Look for a break in the wall pattern
Inspect room edges for a crack that interrupts the surrounding wall pattern. Approach the suspect wall and attack it. A discovery message and a new room on the map confirm that you opened a hidden passage.
Use the pictures as a search cue
The early-access examples below show wall details in a library and a cave. Search the walls in your own layout; these are separate examples, not a before-and-after pair or a guaranteed room in every run.
Confirm the opening before moving on
Step through the revealed gap and inspect the room for its reward. The map can show the discovered room without drawing a connecting passage, so return to the wall you actually opened.
Do not confuse this with Erma’s study
Erma’s hidden study is a separate story location. Players describe a passage after the boss and bridge, to the left of a table, without the usual cracked-wall clue. Do not use that exception as a rule for finding random treasure rooms.