What else should I do to address a mystery leak?

Question

About two years ago I noticed one of my showers was leaking. I didn't have the wherewithal to fix it at the time so l just shut down the shower. I opened up the ceiling recently to finally deal with this project, called a plumber in... and it's not leaking anymore. Pipe looks fine. There are some old wet spots from these photos. I ran the shower awhile, nothing. Threw buckets of water at the shower walls, got my jeans soaked but no leaks.

Plumber says maybe just a caulk/grout issue on the sides that only comes up occasionally. I was planning to close the ceiling while leaving an access panel in case I need to go back in and continue to monitor it. But curious, based on this story and the photos, if there's something else I can do to check for leak source before I put drywall back.

Answer 1

There look like water stains in the blue circled areas:

Maybe the tile floor or where the floor drain joins the tile floor is leaking?

Answer 2

There is some corrosion and oxidation on the trap plug so that might have been the leak and the rust/corrosion could loosen up in time. It's a great idea to install the access panel to continue to monitor this. I'd use the shower for awhile before adding drywall since leaks never happen the way we plan them.