Is this actively leaking? Or does it only happen sometimes, like after someone uses the shower or toilet upstairs?

Shower the in bathroom above? Sink plumbing? Almost impossible to tell. New house? Have you had any work done recently?

Either a pinhole leak in a supply (assuming copper) or a drain.

Try isolating. For instance, fill the bathtub, then drain it and see what happens. If nothing, then repeatedly flush the w.c. If nothing, continuously run a sink.

@beesneeds It's actively leaking. Since taking the picture of the tile area 30 minutes ago, there's now about twice as much water.

" Since taking the picture of the tile area 30 minutes ago, there's now about twice as much water."

Being that's the case you may want to shut off the water and get your plumber in there asap before you have a bigger mess on your hands. Your past the point of self diagnosis I am afraid.

Shut off the water ASAP. Consider it luck you seem to have caught it just as it's starting. Get a plumber in to fix the leak.

Ah June.. kind of difficult to start upstream when the beginning of the problem is noticed at the bottom of the river. And that where the OP is at- noticed the downstream. Thankfully they seem to have caught the whole horror early.

Hopefully the plumber looks upstream for the problem.