This could have been a one time problem from a clogged drain that has been cleared and all that is required is to use a stainblocker and repaint the ceiling.

Or it could be a problem bad tile on the show and the walls have rotted and it will cost 10 of thousands to fix.

Or it could be something in between.

Or it could have nothing to do with the bathroom, but be a roof or wall leak.

You need someone to inspect it and even then they might not be able to give much detailed information until the area is opened up.

"This could have been a one time problem from a clogged drain that has been cleared and all that is required is to use a stainblocker and repaint the ceiling.

Or it could be a problem bad tile on the show and the walls have rotted and it will cost 10 of thousands to fix.

Or it could be something in between.

Or it could have nothing to do with the bathroom, but be a roof or wall leak."

OR Maybe , possibly , could be

it could be the overflow is not tightened properly and water gets behind it.

Or it could be a pitch pocket around the vent terminal needs refilling

Or It could be condensation from the C/W lines leaking during very humid days

Or it could be a joint not properly made and leaks sporadically

Or it could be an heating leak steam vent

Or a heating leak when the hydronic system first starts and as the piping expands the leaks stop like a bad stuffing box on a valve

Or it could be water once in a great while getting down the vent terminal

Or it could be a leak behind the shower body escutcheon when the water is turned on

Or it could be the ELL behind the shower arm connection

Or a defective tub spout where the water trickles behind the tiles

Or a tile stop may have leaked when someone shut it to repair a faucet or shower.

Or like you said a one time over flow

Or someone tried to snake a line and put a hole on the top of the drain pipe and it only leaks sporadically when the waste /soil line is running at full capacity it flows out of the opening at the top

Or a parapet may have a crevice by the copping stone cover

Or the counter flashing is to low and water gets under it

Or then again it could be something entirely else like a wax gasket needs replacing

Or defective soldered joint to the floor flange.

Or tank condensation from a defective Douglas valve

Or a bad tank bulb or flapper

Or a bad ball ****

Or outside walls need pointing

Or the cable guy didn't caulk the opening when he drilled a hole that wind driven rain may find the opening.

Or none of the above I reckon