Bathroom Tub leaking. Concrete subfloor. Help!

We have water coming underneath the floor in the hallway next to the bathroom. We believe the water is coming from a leaking bathtub drain.

This is a single story house with concrete subfloor/no basement. We thought it may have been coming from the roof vent but its not looking that way now.

This is an older home and has a ceramic tub/tile floors in the bathroom. Obviously there is no access panel. The shower wall is tiled.

I'm in a bad spot here aren't I? Does the tub need to be pulled out? Cut a hole in the opposite wall and take a look? Advice?

No way this can be fixed without anything intrusive is there? There is a screw on top of the tub drain.

Have you pulled the cover plate off and looked at (felt around) the valve? That would be my first move.

By cover plate you mean the drain plate in the tub right? I haven't... I'll unscrew that and take a look at it.

I mean right where your tub valve is, the handle to turn on the water. Pull that off and you should have enough room to feel around the valve to see if it's wet.

What kind of valve do you have. It might be as simple as replacing the cartridge if you're lucky.