IMHO, this must be a stolen truck from the Milwaukee company, and outfitted with a Canadian plate.
Does anyone know if something nasty, being 'trucked' into the U.S. via Mexico might be allowed in easier if it the truck had Canadian plates and papers showing it was returning to Canada? And aren't border agents along the Mexican border searching primarily for drugs? What if the truck was free of all drugs, but had something more dangerous that drug-sniffing dogs would not detect.
A story at: http://mccain.senate.gov/NAFTATRUCKS.HTM
says that not all trucks heading north from Mexico have their driver's Commercial Driver's License (CDL) verified, because they don't have the equipment, and even without those checks, back-ups already exceed 4 hours at some border crossings.
The Byers Colorado employees say the truck left the gas station and headed north on '36', supposedly toward Interstate 80. I would think however that since highway 36 quickly bends and heads EAST, the drivers were trying to avoid weigh stations, and are travelling cross country on smaller roads.