Toilets, Sinks, Showers, Tubs and Disposals - Leaking shower.....stumped.

I noticed my shower was leaking when I saw water on the lower wall in my living room. To try and find the problem I didn’t use the shower for 3 months to let it hopefully dry out. I then did the following to attempt to find the problem.
To test the drain I ran water down it from an outside source for 10 minutes, no leaks. To test the pan I plugged the drain and filled (outside source) the bottom of the shower as high as I could, about 5 inches, without it spilling out and left it for 20 minutes and still no leak. To test the incoming water pipes I ran a hose from the showerhead pipe to my backyard and ran the hot and the cold water for 10 minutes each, individually, and at different volume, still no leaks. I am stumped.
Thanks

No plumber or anything, but...

What kind of walls? Tile? Full fiberglass surround? If tile, do you know what it is installed over (drywall, greenboard, cement board?)

50 year old house. Shower is tile, walls are plaster. That's all I can tell you.

Well, unless its been redone..I'd guess you may have leaking through the grout or around a soapdish or similar. You've pretty much eliminated the other things.

Check very carefully in corners and such to see if you have any gaps or staining that might give a clue.

I've checked the tiles and grouting in the area of where I saw water on the wall and I don't see anything obvious. It doesn't help that my eyes are 49 year old. I'll have to check better. I am in the process of doing a thorough cleaning of the shower. Thanks again

Ahhhh one other thing came to mind. Depending on the type of pan you have..it could be bad at a certain point where continual use causes it to leak over time. A small amount leaks into the pan with use and then has nowhere to go, repeat, repeat, repeat, until it finally starts leaking with each progressive use.

A 20 min test would not allow enough to leak into the pan to reach the leak out level.
Hope I'm being clear.

I'd guess the only real way to know is to open up an inspection hole in the ceiling beneath.

Well I think I'm on to where the touble is, the pipes, more specifically the valve stems. I used the shower faucets to fill a bucket to rinse down the cleaner I used to clean the shower floor. I turned the faucets on and off several times over a 10 minute span. I then noticed there was water on the subfloor under the wall that has the water damage (I removed the baseboard). So just to be sure where the problem is I'm going to let it dry again and then do the tests again over a week span. That way if it is, say the the pan, and it's taking awhile for the water to be evident I'll know the real problem.
I'll repost later.
Thanks again

Ahhh jeez, I'm an idiot. That could be the easiest fix of all. Tighten down the packing nuts, or pull 'em out and and replace the packing.

Hopefully taking off the handles and trim won't be that bad.