The individual figures out he's with the "wrong guy" somewhere along the trip. At dinner at the Ponderosa (which I believe is "halal") he drops a note for the cops.
No doubt he will be dead himself as soon as the guy he was with finds out about the note!
That's why Charlie Moose should have just made the call and not worried about who was answering the phone!
Hope that clears up the confusion.
This presumes that the note was found at the restaurant - but it will work as well if the note was found at the spot the truck stopped so the shooter could fire his weapon.
Now we just have to wait to see what the shooter wants to talk about.