Besides the grout, caulk and fan, we look at the far side of the shower wall, any access panel to the plumbing, the floor around the toilet and tub, and the vanity cabinet and its supply lines. Where a wet area lines up with a neighbor's bathroom or kitchen, we record the readings by location, so they can be matched to the unit above or beside you.
Timing helps as well. A ceiling stain that darkens after the upstairs shower runs, or a wall that is wettest in the morning, tells a building manager where to send the plumber.