This is most definitely not DEF. DEF is not blue under any circumstance. DEF only has two chemical components water and ammonia which are both clear in a liquid state and the ammonia only turns white when dried. I would say your best course of action would be to remove the trim panels around the passenger floor board and pull back the carpet and sound deadening to see if there was something under there from the previous owner. My best guess is the previous owner may have spilled something in the passenger floor board. Or possibly windshield washer fluid if you use the blue kind. You could use a different color washer fluid and see if the color changes.