It may be the flashing/collar on the vent stack. Water will find a path, and it sounds like it's running vertically down the stack during a rain. You will have to get on the roof to make an inspection.

I had a funny leak a few years ago. When we took more than three showers a day in my master bath, water would leak into the ceiling below it. I was in process of gutting one bath so we were using the master bath exclusively. The leak really started coming out. So I pushed a hole through the mucky soaked drywall and found a 1-gallon milk jug wired to the shower drain line.

Moral: A leak could be from anything!!!

The drain line was leaking slow drips, so the knucklehead wired in the milk jug to catch the water. The jug sat in the floor space and would evaporate over time. But when we used the shower too much it couldn't evaporate fast enough and overflowed.

Why would anybody go to that much trouble and not buy $10 worth of drain parts to fix it correctly. He textured the ceiling perfectly I didn't even know it was repaired.

I'd like to see the previous owner some day, I have a few choice words for him. I took lots of pictures as I opened the hole in the ceiling, I was hellbent to sue him, but my wife brought reason to the situation. When he sold the house he was mortgaged for more than what it was worth, so why would he have any more money today.