Sorry in advance for this long scenario for a hopefully simple question. The main one is: can there be an almost-steady leak from a PVC vent pipe in a bathroom that's not operational?

We are building a new bathroom on our first floor partially below the footprint of a second-floor bathroom. We had assembled the exhaust/vent pipes using PVC pipe that goes up through the ceiling and on into the attic, where it connects via a Y-pipe to the main vent pipe going out to the roof.

After we had finished installing the ceiling and started work on the finishes, I noticed discoloration in one area. I broke away the ceiling and found extensive dampness at a point just underneath one of the PVC pipe joints. (There are five elbow joints or connections in this area - it's where the exhaust from the shower and sink meet up and then bend upwards to go into the attic). The dampness deeply saturated two pieces of wood - a joist through which we had put a hole that the PVC pipe runs through, and a 2x4 acting as a cat brace between two joists (and which ran directly beneath the PVC pipe, both parallel to the ceiling). I assume some white stuff now on the wood is mold.

Here's the problem: It was really wet - at one point I placed a plate on the floor below because it was collecting a steady drip. But it was intermittent and there was no clear pattern.

Here's a twist: I got up there and wrapped some plumbing pipe tape around the joint just above the cat brace - not cleanly. The cat brace was so close to the pipe that the wrap wasn't tight at all. But since then, I haven't had the leak come back.

Here's the mystery - the bathroom was not yet operational. No fixtures had been attached. I can't figure out how the vent pipe for a bathroom that isn't being used can cause that much condensation or excessive wetness. (This was in December/January). Is that possible?

I wondered if it was possible that there was a leak from the upstairs bathroom but there is no sign of it from the floorboards above nor any pattern I could re-create (leaving the faucets upstairs running or dripping - none of it led to the leak). We even had big rainstorms before and after (rain coming down the vent pipe opening?) - but no pattern.

I think the easy solution is to remove the cat brace, really tightly wrap the pipe, replace the brace, and then move on. But I want to be sure before I close up the ceiling that the leak was coming from the vent pipe, because I don't want it to return after we have all the fixtures in, etc.

So the questions are – (again) can there be an almost-steady leak from a PVC vent pipe in a bathroom that's not operational? Will I be safe if I just wrap up those PVC pipe connections/joints or do I need to re-glue them? Should I keep looking for a leak from the upstairs bathroom?

Thanks for any help you can offer.