Most of the time, it's the putty drying out on the drain.
Normal repair is to replace the drain, using Silicone this time from the underside. Yes, the ceiling will be cut out to do this.
If not the drain, it could be someone showering with the door open, ( Yes, this really happens, they sometimes don't admit it until after I've cut holes in their home. )
Or one time, it was a broken shower arm. I discovered that one in a third floor apartment, they knew the shower head was dangling there, but didn't bother to tell anyone.
Or it could be the seal around the shower faucet pie pan.