Strange problem, super slow leak?

I have a weird problem that I have never encountered before. I have Falken Wildpeak AT trail tires on my Forester. They have about 45k miles (warranty is 65k if that really means anything). To me the tread still looks ok, and my mechanic agrees.

The problem is since about December, one of the tires has a super, super slow leak. Noticed it on a road trip. It will hold air, but not like the other tires. I thought maybe it was the TPM sensor, but my gauge agreed it was low. It is fine around town, but only has a problem with prolonged highway use (sometimes it will lose a few PSI over 300 miles/5 hours), sometimes it's faster. I fill it with air, but it never seems to want to hold more than 32 PSI (otherwise it does down a bit faster, and then stops there). While I was recently away, I left it with my family's trusted mechanic (been going there for 40 years). They checked the tire and couldn't find any leaks. But then driving home, it slowly lost a few PSI compared to the other tires. I know at some point I need new tires (and since I have a Subaru it's 4 new tires at once). Could this be a slow leak at the bead? Should I take it to a tire place and have them remove and remount it? I hate to replace the tires when they still have some life left (and drive ok). Just a weird problem I have never encountered before.