I see a spot below the copper pipe, that looks like it has water on the wood. I would have a plumber come and check it out, before it becomes a major problem.

Could be just condensation, but that elbow looks suspect too.

Yes exactly, the marks on the Y is the reason I posted this. Yes there is a toilet upstairs, only in one occasion we have seen a little water on the floor downstairs but we couldn't figure out where it was coming from. I suspect if there is a little clog upstairs, the water leaks from the wax ring upstairs and drains down on the pipe (or maybe it happens when we engage both flushes together).

Thanks for the replies. I'll do a couple of tests to see if I can confirm the diagnosis. Hopefully just need a reset of the toilet upstairs.

Yep a new wax ring is a pretty easy fix.

If you have raised the floor (installed tile, for example) you may need a specially made ring for that.

I'm assuming that the toilet is very close to that stack - not several feet. If that's not the case, I'd look at the roof boot (assuming it's a straight line to the roof). If you can see daylight when looking up where the stack goes through the roof in the attic, it's the boot.

Ah, I see, it's gonna be tough to look in that direction in that little space, but I'll try; thanks for the tip. Yes the toiled is very close to the stack. Now getting excited to fix this thing! :D

Since it's close to the stack, I'd say the wax ring is the easiest and most likely.

Start there.

If you still have a problem, then keep looking.