Is the trap/drain covered?

Take an ice pick and start poking at all the studs and joists around the units old location.

The wood should still feel solid, with the pick going in note more than 1/8 to 1/4 inch with a moderate effort.

Start high up on the wall to get an idea before going down near the base.

Places especially to check are seams in the unit and the floor around them.

We tried your suggestion, using an awl, and found no indication of soft or rotten wood. Everything was solid & the awl didn't penetrate more than 1/8" anywhere. There is no discoloration on any of the joists, studs, or plywood.

+1.

Is the trap covered?

You could be getting sewer gasses back into the home if it has dried out.

When we tore out walls in our 1920-vintage house there was some behind-the-walls mustiness that came out, just because the dust inside the walls is old, I suppose. It eventually dissipated.

Is it a dusty musty smell? Or a wet/sewer musty smell?

Thank you for your suggestions. We think we have located the problem - a combination of a bit of water damage below the drain pipe combined with air flow after the tearout. We will make sure all is corrected before going forward with our project.