I am debugging this completely remotely from the actual house :(. Tenants talk about a damp ceiling corner and were originally scared about the 'bowing' in the ceiling BUT after they showed me a picture with two vents and one vent had a gap, taking the vent off revealed a crappy job on patching the drywall. However, I am wondering if that was there from some leak previously.
I told them to wait three days with the shower above that one off(since it is guest bath and they don't use it much) and then check if dry again.
I realize this is probably not enough info so I am generally looking for random ideas before having someone drill a hole and actually check if the other side is wet. If we do a drill a hole and it's dry, some combination of using the shower every day may need to be tried (isolating leak to before or after the turn-on valves).
Any other ideas before we have someone cut into the drywall are very much welcome or is this pretty exhaustive?