on the trailer or on the cab?
I was behind the things and where there was supposed to be the licence plate there was a an empty rectangle.
Military moves weapons and nuclear weapons on regular trucks that are licensed to look like regular trucks (incognito).
Distractor vehicles. Cops pull those over and search them while the real trucks carrying drugs/humans roll on by.
Or they're just incompetent owners.
Often on big rigs, because they change trailers so often, the license plate will not be put on the trailer, but instead will be put on the front of the truck. It is more important that the actual rig have the license plate instead of the trailer. So if you are driving behind a big rig, you might not see a license plate on the back. Not a big deal.
Yes, but OP didn't specify if there was a trailer or not.
Military trailers don't have plates but usually have a 'bumper number'... an identifier of the unit it belongs to and the vehicle number. These can be obscured or removed when a unit is moving for security reasons.
Government vehicles have the 'white plate'... plain white plate with black or blue alpha/numeric markings.
And, yes, tractors typically have the plate(s) on the front so they can be ID'ed when they have a trailer on (going across a scale, etc...)