Could be a leaking tub drain, or drain is rusting out.
Could be water getting past the shower curtain.
Could be water leaking past the caulk at the tub edges, or missing grout at a wall joint.
Could be a drip behind the wall from the tub valve.
The seal on the tub overflow could be bad, and water is splashing onto it then getting behind it.
The escutcheon plate on the tub valve could need caulking, or needs caulking around the tub spout.
The toilet might have overflowed, or the wax seal at the floor is leaking.