Plumbing Issue - Any thoughts?

[Boots Bonner]

I have owned this townhouse for 7 years. My tenant tells me that when someone is taking a shower in the upstairs bathroom, there is a drip through a vent on the main floor. The vent was used by the previous owner to provide access to the upstairs bathroom plumbing. 

When I called the handyman, they could not locate the leak.  My husband stood in the bathtub and ran the water from the faucet, no drip downstairs.  Any thoughts?  Could it be from the shower itself combined with the weight of the person? 

[Bob Jaeger]

@Boots Bonner Attach a hand-held sprayer where the shower head is currently and spray systematically from low to high on the control wall for a few minutes in each location. Have a helper watch from down below. Start at the level of the tub overflow, then at the caulk line where tub meets the wall, then above tub spout, then the shower control valve. The person watching down below will know when you’ve found the area that’s causing the problem. Sometimes caulk is the correct solution. Often though it’s more involved. Feel free to message me for more help.