Damp concrete slab in bathroom
In our guest bath, we had linoleum as the flooring. The floor around the toilet has always been a little darker than the rest of the floor, so we didn't think much of it, but then over the past few weeks we noticed that the darkness is spreading.
We pulled up the linoleum, and discovered that the floor underneath is damp. The bathtub is right next to the toilet, and we ran it and the shower for a while to see if it could be causing the leaking, but didn't see anything.
So, we're stumped as to what could be causing this. The dampness looks like it's spreading out from the toilet, but it's just wetness -- not excrement (so not like a failing wax ring or something). Any guesses on what could be causing this?
We've pulled out all the flooring and the toilet. If we let it dry completely, and then run the bathtub/shower for a long while, and no wetness appears, can we make a safe assumption that it is NOT related to the bathtub?
Also, who would we possibly call to help us troubleshoot this (and presumably fix it if there is a problem)? A plumber? Someone else?