Problem is isolating the leak - with a sink, toilet, and shower right above, it may not be easy...
So we tore into the ceiling and found that the dripping started each time we...
oh, man...
each time we flushed the TOILET! Yuck!
Fortunately the leak was small, and hadn't been going on for that long. Removing the toilet, we found that the basin had shifted, and messed up the wax ring.
$2.79 and 20 minutes later we were flushing again without hesitation. No serious damage, quick and inexpensive fix, and best of all, NO smell! How often does THAT happen? Not often!