I have a shower drain that was snaked a few months ago and has started having a very light back up again. After maybe 5 seconds of unenthusiastic plunger work, it clears up just fine. But come back an hour later, run the water again, and it backs up again - even if no one has actually used the shower.
Draino and the like haven't resolved anything. We snaked a second shower at the same time, but haven't had a problem with it since then. No other drains seem to back up. We also got an external drain cover to capture more hair, etc after the snaking to try to keep the clogs down so I would hope it's not more hair.
So it's making me think maybe something with the waste line for this shower? Are there some ways to test this? If that is the problem, is there anything to be done without getting into the walls?