Help me find a leak?
OP here. Just to circle back to provide an update: on further exploration, I now believe I'm dealing with an exterior leak somewhere around the cap. I've identified a couple of spaces where the original caulking may not have been applied evenly and might be a place for water intrusion. Looks like a full recaulking around the cap is in order. Seems like the simplest explanation and hopefully the correct one. We'll see. Thanks for your attention!