is it where the force of the shower head hits the opposite wall?
I am sorry that you do not have a one piece stall, but that is now irrelevant.
Anyway, if you have caulked the seams, and if your leak is not in the connections of the water supply and handles, I would then certainly say its the union of the drain into the sewer line. Which, YES, means to get under the pan.
Many pans are so different. Some are just sitting on a wooden frame, then some are made of sculptered concrete as to design a bowl with the drain in the center.
These leaks in this area are a real pain and so hard to isolate unless you pull everything out.