It kind of looks like somebody tried to do a DIY repair to that area, I can only guess that it leaked on the previous owner, and that was their attempt to fix it, it looks like a big blob of solder, it may have rusted so they had to hog out some of the area, then try to fill it.
I could be totally wrong though, just tying to guess.
It does look like water was taking that path though, you can see below the blob/hole where dust/dirt has stuck to the area, I have seen that on my own car where leaks are present.
Hopefully that area stops leaking on you, let us know how it goes.
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA