DIY & Home Improvement

are you comfortable with dry-wall repair, or know someone who will show you how?
You've got to get a better look behind the walls to find out where the leak is. Small holes about 3 inches square are easy to patch, but replacing a whole sheet is easier than trying to patch more than 2 holes.
Keeping all that in mind, I would start at the tub (the wall behind it, where the feed and drain pipes are).

before that though, did you try filling the tube up past the overflow valve? if the seal there is leaking that could be why it only happens sometimes.

first fill up the tub halfway. let that sit for a few minutes and go check if you see any leaking from the hole in the ceiling. if so, it's the tub drain leaking. if not, fill the tub almost to the overflow valve, check the ceiling again (more water in the tub will create more pressure, and might show the leak.) if not, fill it more so the overflow is draining the water, and check again. if you still don't see a leak, then it's probable in a pipe connection in the wall and you will have to start pulling drywall.