The corner of the bumper has a liner pin above that area and the hole it goes in is quite a bit larger than it is. I would assume the water that is collected in the trunk galley runs close to that because it's behind the tail light.
I had a leak in the same area a while back. That's where I thought it was coming from, but it was actually coming from above where the tail light pan is pinch welded to the water galley around the trunk. The seam sealer had cracked.
Just to be safe, I put some seam sealer around the liner pin as well.
I hope you get it straightened out. It was a PITA for me to find mine.