Around the rest of the tub, there is probably what is called a tiling flange. Before the wallboard is put up, they should have installed a vapor barrier - roofing felt or plastic, that came down and lapped over that tiling flange. Neither the tile nor the backer are entirely waterproof.
If the tub doesn't sit exactly level, any water that might get behind could be channeled around behind the wall and appear there. BTW, that lower skirt may in fact be a separate panel.
Sometimes, when you add the weight of a person and a tub full of water, the floor deflects, and opens up a crack that may not be obvious when it is empty. This is also the best time to caulk things, since the crack is the biggest it is likely to get.
You could also have a leak from the new plumbing, assuming that was changed before the new tile was done, or in the process, the old stuff was disturbed. It could be coming from the shower arm's connection in the wall. Sometimes, you can pull the eschution back enough and look in there with a flashlight.
If you don't have an access panel where you can look at the back of the valve and plumbing, and it is where you could add one, you may want to cut an opening there and look to see if it is plumbing or a leak through/behind the wall.