The sauna appears identical to all the others at first glance, but one of the ravens will fly in and peck at one of the stone bricks in the wall. Examining the stone, it appears to be slightly loose.

However, there's a chance that disturbing it could cause more trouble than it's worth. Astarion and Haelva can decide to loosen the stone and see what it does, or they can choose to take precautions, find someone who knows protective magic, and return later.