Have a 6x6 wetspot on my concrete slab in the closet behind the master walk in shower

You should probably open the closet wall to see what's leaking. It could be a supply pipe or fitting or a drain.

Brian has the right idea. You must determine if this is a hidden leak under the slab or a leak from the shower. Hold off on tearing the wall open and try this. Close off the shower drain and fill the base with a few inches of water. At the same time direct the shower stream at the tiles the stream would hit and keep checking the closet floor for moisture, If no moisture shows up shut down everything in the house and go outside to the meter. Is the little pointer creeping or moving? Get back to me with the results and we'll work on a solution. Good luck, Tom