Did you find a fix? I have the same problemHasn't been hot at all lately.
Condensation would be one explanation if 1- it didn't happen after each rain, 2- it wasn't at least 1/2 cup or so of water being held there.
I totally dried the area last night, and today had the Treg washed. After the wash when I opened that door to get a better look, some water dropped to the ground. It seems like the water is getting in somewhere at the top of the door seal and getting hung up between the bottom of the seal on the door and the seal on the side of the door sill. Just gotta' figure out where it's getting in, and or why it's getting held in there.