If the receptor was built properly it has two concrete slopes. The first one on the subfloor, then a vinyl pan-liner, then a second slope.
Water will get through any tile and grout. This is why the floor slopes, obviously. Under the top slope is the pan liner and it too should be sloped to the drain as it sits atop the sloping pre-sloped concrete. The drain should contain weep holes that would take the normal water getting through the tile and grout to the drain.
It is safe to assume that wherever the leak is it, is not in the area (above) the top slope and liner. This could mean the liner was improperly installed and usually this infraction is because the liner was nailed to the studs too low and the nail hole(s) are leaking or the corners of the liner were cut instead of folded.
The walls themselves could also be leaking and directing water behind and below the upper-most folds of the liner.
Not IF the pan is sloped to the drain properly.
No!
Here's the next thing...
If the receptor was built without a pre-slope as a lot of them are then the water that migrates into the pan below the tile is just sitting on the pan liner and going nowhere. That water would exchange itself to some degree when the shower is used but would then sit/pool once the shower turned off. If this is the case the water could be seeping under the drain device and finding its way back away from the drain slightly as it clings to the bottom side of the pan liner. This would cause the leak to appear to be at the drain when it is only near the drain and not the drain itself that is leaking.
Run some more water and see if any of this is transpiring.
Not that it matters now, but, how old is this shower?