Shiera Seastar
Banned
Yea, but you can also find them by shooting at walls with guns that have finite ammo (so not starter weapons) to see if it starts breaking. Beyond that, I still don't know if there is a way to figure out where a secret room will be or if they even spawn outside the first floor.
I usually find a few every run solely by checking the walls in chest rooms & and the northern wall in shops where sometimes extra merchants hang out. Those are definitely not the only places a secret room can spawn in though.
It's cool when you find one but honestly, I'm not totally convinced they're actually worth a blank. You might get a key or armor plus an extra blank but often times you just get a heart you don't need or a locked brown/blue/green chest you might not have keys for. If you're low on blanks & haven't fought the floor's boss yet, it's a gamble. Coming from Isaac it's easy to be fixated on secret rooms but a single blank in Gungeon seems much more valuable than a single and easy to replenish bomb in Isaac.