About a year ago we started seeing a small water stain in our downstairs ceiling in one spot right underneath our shower/tub upstairs. I tested some theories and decided in the end it seemed to be a result of bathtub splashing by my daughter. I put silicon between the tub and the tile and did a shoddy drywall patch on the wet spot. We lived in bliss for a year until... about a month ago the spot reappeared.
I remembered that around the time we dealt with this last year and again recently; we had had some tub backups(I love my wife and daughter but their hair causes me a lot of work). I tried snaking the drain and got the water moving decently but the drip from the area got worse.
We switched to using the downstairs bathroom for the last two weeks. Today I cut a hole in the closet drywall to see where the drip seemed to be coming from when I investigated two weeks ago. Then I turned on the water and there was no drip; not when I filled and let it drain, not when I ran the shower; no drips.
My current theory is that where a connector and pipe meet of the vent pipe is where the leak is but it only happens when the water backs up at a certain point(after the junction of the vent with the bath pipe). So a backup more towards the tub causes no leaking. But on the bright side, now that I’ve got my hole in the closet wall I just need to wait for the leak to return.