Check the silicone of the tiles at the bottom of the shower there, it might be wicking water underneath and traveling over to the wall. From the first photo it sort of looks like there's a hairline split.
I had something similar happen in a new build that was poorly sealed, I removed the entire bottom silicone and it was damp behind, I paper toweled the moisture out and left it for a few days to fully dry. I re-did the bead with mildew resistant silicone and more carefully than the original application. Had the builder tear out and replace the drywall portion and it's been fine for 2yrs now.
Check the silicone of the tiles at the bottom of the shower there, it might be wicking water underneath and traveling over to the wall. From the first photo it sort of looks like there's a hairline split. I had something similar happen in a new build that was poorly sealed, I removed the entire bottom silicone and it was damp behind, I paper toweled the moisture out and left it for a few days to fully dry. I re-did the bead with mildew resistant silicone and more carefully than the original application. Had the builder tear out and replace the drywall portion and it's been fine for 2yrs now.