Helen-
In the old days (1980-81), these "little black boxes" were known as "shakers" that detected motion or vibration. They could be jerry rigged anywhere on the truck including the chassis or engine compartment.
The shaker I found (1980-81) was disguised as a package with our customer counter shipper number on it. At the time our state didn't have intrastate shipping rights so it was a dead give away. I sheeted the package as missed and handed it to the center manager and told him he could give this back to LP.
As others have mentioned, just do the job, pace yourself because you have to do this all day and be safe. The cemetary is full of heros.
If you aren't doing anything wrong they can put antenna's, mirrors, shakers and a telephone capable chimpanzee on top of your truck.