Probably a condensation issue but might be from higher up. Was looking at the inner arch on mine next to the fuel cap and this area is starting to corrode its pretty much same area but 3 door model.
Once I get chance I will remove my interior carpet at side and the trim as I need to figure out if its going to be something I'm able to tackle.
maybe just needing to change out the fuel filler cap and tidy up the arch in my case.
You should check this area from the outside and see if it has similar issue.
my problem water inside was caused by the boot switch surround but was quickly sealed after the switch was replaced it was dripping down into the wheel well last year