I can't see the pictures, might be my machine. But, sounds like you need to dig around a little more. Can you get to the subfloor under the shower? Might be a good idea to take a look there. The pan might be leaking. There are a couple of ways to check...assuming you can get the grate off of the drain, get a long party balloon or buy a drain plug made for showers. Ensure the plug is at least 4" or so down the drain to ensure you have the weep holes covered, and inflate it fully. Fill the pan up with water...mark the level and let it sit overnight. If it drops any, you have a leak in the pan. that is, assuming it actually has a pan. Older showers had them made out of lead. Newer ones are often a pvc membrane. Lots of things can cause them to leak (they last a long time installed properly, though), if they ever were watertight. Note, a common fault in installing a liner is to put it on the floor. While this might work for awhile, it inevitably leads to a smelly shower. The liner needs to be applied over a pre-slope so any moisture that gets below the tile (and it will), can flow down to the drain rather than just sitting there and becoming a swamp. My unprofessional opinion.