Is my shower drain leaking under the slab?
It's possible the water is getting around your shower, possibly where the tile meets the tub or pan (if you have one), or behind the fixtures, or even around the door (if any). 

I would check all of these and recaulk where appropriate before going in for wholesale destruction. Try splashing water by hand/bucket on areas you might suspect and see if that leads to leaks.

Or maybe you even have leaky plumbing in the wall. Do you have access to see in there?