Could it be some sort of chemical such as Suntan
oil, deodorant or perspiration? Or somthing that may have leeched (?) out of any clothing when they got wet on the trip
Or was there some type of leather dressing originally applied on the seats that bled to the lower part when they got wet?
Go back and look at the outerwear that you had on (if you can recall) see if there are any clues on the jacket etc.
After affects of Chernobyl Rain..........