When the UPS guy walks into our office, nobody looks out the window to see if he came in a UPS truck. Plus, he would have to get through the metal detector and the deputies at the door first. Even the employees have to go through the metal detector.
Excellent point! He wouldn't necessarily need a truck.
He could drop off a package and leave and detonate it from across the street.
You know those hand held devices they carry that they get you to scribble your name on?
That could be the detonator.