How should I investigate an exterior leak from bathroom shower use?
You can do some investigation without opening up a wall:

(1) Fill up a  2-gal bucket at some other faucet in the house or outside. Pour this into the shower and then look for the water leak you've been seeing. If you see the leak, then the leak is in the shower pan or the drain.

(2) Run the shower into the bucket so as to catch all the water and none goes down the drain. If you see the leak, then the leak is in the water supply past the shower valve.

EDIT It might take more than 2 gal to get the leak to show. You might have to use two 2-gal buckets.