Originally Posted by Evstarr
In that case, I would have to say that unless there is a can light below you can remove to see the area that it's time to pop a hole in the ceiling. This will eventually have to happen anyway if that spot keeps getting wet.
As he said, your eventually going to have to put a hole in the ceiling to fix the water spot. So open it up now. BTW most tub leaks show up under the drain because in order to fit the drain there is a hole cut into the subfloor. Any leak above eventually reaches the hole.
Once open, turn on the water in the tub. From below look,feel and listen around for any water. There could be a leak in the pipe from the fixture to the spout. This happened with our shower the leak was spraying water on the outer surface of the shower enclosure and it flowed down the side and evetually was dripping off the bottom of the trap.
Place a bucket under the hole and fill the tub. Check if there is water in the bucket, wait 15 minutes and check again. Start looking and feeling for the leak.
Another trick is to get duck tape and tape around the bath tub drain, but don't cover the drain holes. This will create a temporary seal around the drain. Fill the tub and wait 15 minutes. If that stopped the leak, then you need to replace the seal around the drain.
P.S. feel lucky you already have the spa tub. We had a similar leak, once I started knocking holes in the walls, the wife decided now is the time to remodel the bathrooms. $8000 later and the shower doesn't leak