I have a very bad feeling about this and plan to call the same plumber in the morning. I had just washed my hair, taken a long shower and cleaned the 2 toilets upstairs. I waited a few minutes then flushed one by one and turned on individual taps/showers etc... No leaking, but there is no mistaking that moisture around the vent.

I can't pull the vent off to see how much water damage there is.. any thoughts on what this could be or what questions to be sure to ask my plumber when he comes back to inspect the damage?

1. Check the ambient relative humidity. Laundry rooms are notorious for heavy concentrations of moisture build up especially without outside air vents to get rid of this build up of excessive humidity.

2. Check your filters both on the AC units and the drier flue piping discharge.
3. Think about a dehumidifier in your basement/laundry room area
4. When was this flue pipe vacuumed out?

A lot of leaks can take days to locate the source even months in some instances.

Unless the leak is from a water supply line they are hard to find. Often we use trial and error.

For example if I were to THINK it was your toilet I would ask you not to use this one toilet for several days and then see it the leak stops.

This "leak" could be just simply the toilet tank loosing water slightly and the condensate forming on the out side of this tank is making its way to the floor below through some hair line crack in the grouting behind the bowl.

The plumber is not in your home 24-7 and therefore he must rely on you also to be on the look out when this occurs.

I have found 97% of the time the house wife has a darn good indication of which fixture is causing this problem.

All it takes is trial and error and keeping the frustration to a minimum.