That may have been a tough one to find without all the rain. Rather even a megger would have found it in a dry condition or not, I do not know. I had a similar situation few years back. We upgraded a fire alarm system and once in a while, it would get a ground fault for a while and then disappeared. We chased it for weeks. Many times by the time we get there and the fault would be gone. If it was still there while we were there, it would go away while we were working on it so we had no idea if what we did had anything to do with it or not. It was fustrating for all involved to include the person who worked next to the panel.
I eventually speculated the one of existing exterior horn strobes were the culpret. We live in middle of a rain forest. Everything here is wet. Once I convinced everyone that had to be the problem, We replaced them and the problem went away. Sometimes it is the weather plays a factor in finding the problem. I have similar experiences with underground wiring. If the problem is connected to the outside, I ask about the weather. You may get dumb looks but it lead you to the source of the problem...
