Just because you can't see it nor touch it it doesn't mean it doesn't exist.
As part of my business I do a couple of 'Z Checks' of Boeings or turbo props each year where we totally strip all components and then cut up the air frame for scrap.
Most of these aircraft will be 25 to 30 yrs old at the time.
The avionics bay insulation and underfloor ducts, wiring and cabling will usually have a sticky to the touch thin film of 'something' but it smells like engine oil , it's so uniform it looks like it's been sprayed on , these are in extremely unaccessible areas of the airframe which would rarely be viewed let alone cleaned during a major service.
To the curious Boeing 727 + 737 classic pilots and engineers have a look behind the the forward rack in the avionics bay with a maglite (near the relay bank) and to where the walls of the avionics bay meets the bottom of the floor and you should be able to see what I am talking about.
If you ever knew a really heavy smoker who liked to smoke with the windows closed in the car it's like the sticky tar that gets left on the inside of a car's windows.
By comparison cable runs and other components in the top of the fuselage (behind the ceiling panels) still look like new 30 yrs after installation , clean !
So there is some evidence as the aircraft get's older (due to components not working as well as they did or less than optimum maintenance) contaminents are entering the pressure compartment and over a period of decades you can see it in some parts of the aircraft.