On Monday, May 1, 2017 at 5:51:19 PM UTC-4, wrote:
WD-40 does not completely evaporate over time unless the temperature
is higher than that which is compatible with human life.

No, it does completely evaporate. That 'leftover' is the previous skunge dissolved and spread over everything.

To prove this out:

Take a standard paper towel, clean and dry.
Saturate it in WD-40.
Leave it on a clean surface in a location in your domicile exposed to normal temperature variations.
Check it in 4-20 days depending on ambient temperatures.

Don't take my word for it.

Peter Wieck
Melrose Park, PA