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?