The note can be found at Wade Airport, inside the security gate hut by the tarmac entrance, on the counter by a broken terminal.
Just a friendly note from the daytime guards. It's probably not a good idea to let the mechanics bring their personal vehicles through so they can work on them during work hours. Whatever they're giving you in return, is it worth losing your job over? By the way, this is an airport... there are cameras everywhere. We all see you.