[oh'mike]

If there is a hole in the concrete,I'd put my money on moles or gophers.

This is a rare but not unheard of problem under bath tubs on a slab.

 

[vsheetz]

If there is a hole in the concrete,I'd put my money on moles or gophers.

This is a rare but not unheard of problem under bath tubs on a slab.

I had a large amount (garbage can full) of dirt under a tub when removed in a recent remodel activity - moles...

 

[DangerMouse]

Almost assuredly mouse nests from your description.

DM