follow the tiny ants
Here's just an anecdote, of what worked for me.
For weeks, I've been tracking down a series of small leaks on the roadside front corner, before I start to repair the floor. Several times I thought that I'd cured them all, only to find after a heavy rainshower (no light wetting down with a hose) that there was dampness in the floor channel as evidenced by water stains on the paper towels that I laid in there.
Sooo, yesterday I was up on the ladder inspecting for the umpteenth time, when I saw some tiny tiny ants. They were ants that were still infesting a rotted section of floor that I will cut out. What could ants be looking for, up there on the roof, in the hot sun?
So I rapped on the roof with my knuckle, and watched the ants scurry back down their ant trail and then disappear underneath the end of the awning rail! I'd done the rails very carefully before on the top seam, but I was amazed. So I caulked underneath from the end of rail backwards for a foot or two, and then the next day I gave it a good hard wash with the jet setting on the hose. The leak has been cured, finally!