Very right corner of the pic, on the wall, there's an access panel. Should be four screws holding it. Pull those off and it should reveal the back of the shower faucet. Check to make sure those water lines are tight to the faucet and that the connections aren't cracked. Then, water test it. It could be the faucet leaking internally- they do that a lot. It could also be the shower pan or drain leaking. You should be able to see a lot from pulling that access panel off.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR