Any storms that have caused flooding in parts of the USA could have caused a problem. CarFax is questionable as to whether or not it will give you a straight answer. I had a vehicle totaled and it sold a month later with a clean Carfax by the dealer that repaired it. Carfax info requires someone reporting to them. No report, no info. There is a HUGE incentive for dealers NOT to report damage to Carfax and cut the value of their vehicle.
If the rust is more than you expected, than the cause is irrelevant. Keep in mind that if the rust is that high, there are electrical wires and connections that may also be affected.
