Agree with nosticks...check the side seam above the driver and passenger door. My FE did the same (although nowhere near a gallon. That’s a LOT of water!). In my experience most leaks are not too hard to spot but they often are not where you’d think they “should” be. If you have lots of water behind a panel it will flow unpredictably to the low point. Water can do crippling damage seemingly overnight. I’d spare no effort in getting this fixed. Good luck!
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