Disturbing wet patch on carpet

I came home this evening, and the odd patch by my door that I had thought was just a pressure mark in the carpet turned out to be wet.

I have no idea where the water is coming from, but there is a patch the size of my hand, just inside my front door where it has leaked from *somewhere*.

The rental office doesn't have a clue where it's from, but I talked to my landlord (hoorah for living in a small building) and they'll either come by tonight, or in the morning and look at it. Which will probably involve peeling back the carpet to get at it, but since I don't want my floor to squish as I walk, that's okay with me.

I just hope it's something easy to fix, instead of something that requires tearing up much of my flooring.