GFI Breaker tripping

Hello, I have a ranch house on a concrete slab with 100 or 110 service (I can never recall). There is a breaker in the box that services several wall outlets as well as the garage door and associated lights. This breaker is a GFI breaker with a test button. Frequently, during a medium to major rainfall, this breaker trips. When I reset it, within 5 seconds it trips again. My best guess was perhaps the electrical wiring in the garage attic was being exposed to water due to the storm. I investigated as best I could and it looks dry. No usage of a faulty electrical device caused this tripping either as it first tripped in the middle of the night at around 2am when pretty much nothing is on. Do you think I should replace the switch or better investigate if the electrical is being exposed to water? Any other ideas are appreciated.

 

The coincidence of the rain and the tripping seem to warrant more investigation. Are there any exterior devices on this circuit? Yard lights, low voltage walkway lights, exterior outlet for string trimmer, lamppost by the front walk, outlet in crawlspace, lights on the deck, by the pool, water leaking through the roof to a garage walkout light, water running down the wall of the garage? It seems as if it is soaking through something such as insulation, siding, box exterior before it reaches the circuit to cause it to fault.

Hope this helps.