Boiled frog syndrome, tiny whiff of fuel which gets worse extremely slowly. Everyone gradually gets used to it and accepts it as normal.
You can sometimes tell which aircraft a pilot flys by the type and location of the stains on his clothing. Westwind Pilots typically have a large carbon stain on the upper arm/shoulder area from reaching into the engines to depressurise the thrust reverser. Metroliner Pilots have lines of oil from contact with the edge of the gear doors. High wing Cessna Pilots have a gas stains from drips from the fuel drains.