I have a GFCI in the garage that tripped and keeps tripping. I traced it to an outlet in the basement but there is something weird going on. I'm not an electrician, but I've done a bit of wiring, but I don't know how to interpret this.
In the picture is the basement outlet pulled apart and the power is on and the GFCI is reset and working. This basement outlet has 3 14/2 cables coming in. I think one is power from the garage, and the other two lead to outlets outside the house. I checked the wires until I found 120V and then marked them with yellow tape, which is what is shown in the image.
However, if I connect the multimeter to the black/yellow and one of the other whites I get a reading of 101.8, and the other white reads 0.7. This shouldn't be happening, right?
The electric gremlins and the electronic gremlins have a nasty habit of striking once and then crawling back to their lair to strike another day lol. It makes debugging problems much harder if the problem "went away" on its own.
But in any case, your description of a wet outlet box could be consistent with an intermittent fault. If your outlet was oriented sideways with the neutral (larger pin) facing down, it would only take maybe 1/4 inch of water trapped inside the box to short the neutral screw to the grounded case.
It's also worth noting that GFCI receptacles don't last forever. The nicer ones trip themselves off for the last time when they fail their own internal self-test, never to power back on again. The cheaper ones might become oversensitive with age, tripping for really tiny, harmless current diversions. But 8 or 9 out of ten times -- I'm spit-balling -- recurring trips are due to a real issue in the circuit, rather than GFCI.
Old equipment with motors, such as a fridge, are good candidates for leaking small currents to ground. But it's usually maybe 2 mA or something small. You'd need other appliances that are also leaking small amounts to reach the 6 mA to trip the GFCI.
All in all, since you observed tripping with no appliances plugged in, and I assume all the wiring is readily visible and you've already inspected it for any physical damage, that leaves the outdoor receptacle as the most probable cause.
You might consider looking at how the outdoor outlet box would drain water if it did accumulate inside. Most of those boxes are meant to be water tight, but that also means if water enters, it has no way to exit.