Any chance the previous owner attempted some "amphibious" driving at one time? If water were allowed to reside in the floor area, even vigorous water removal might have missed some areas that were allowed to retain moisture for an extended period?

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car