AirTags aren’t designed to track something in movement; this isn’t like a Tom Cruise movie, where spies track a car in a city, seeing exactly where it is in real time. They are meant to be used to find lost keys, luggage, or other objects. But my experiment shows that you can track these devices to a certain extent.
The reason for this is the sheer size of the network of iOS devices that can locate AirTags. Apple says that there are nearly one billion iOS devices around the world that participate in this network, and that ensures that you can locate AirTags in most situations.
I don’t know if any of the truck drivers carrying the mail didn’t have iPhones. Even if they didn’t, it’s possible that if someone in a car driving next to the truck has an iPhone, then it would be spotted.
After the AirTag was delivered, my friend left the envelope on a table in his house. He has an iPhone, so I expected him to be notified of the presence of the AirTag after a while. According to Apple, anyone who is in the presence of an AirTag that has been separated from its owner for three days will get an alert on their iPhone. They are supposed to get an "AirTag Found Moving With You" message.