they do not need to inform you ... as they can be used as a way of lowering insurance on the vans...

have had them fitted to ours for 3 years.

they register time ignition is turned on ..
time engine started...
time of actual movement..
time taken in traveling ..
route taken..
etc etc etc..

now in our eyes were that we knew it was to keep an eye on us so we all slowed down ,basically losing a job a day.
we kept it up for a while , and basically turned out one of the bosses was sure one of the engineers was extracting the wee on overtime ..

but the upside was that it required a dedicated member of staff to moniter all the data.. and no one could be bothered after the first 6 months

but what you can do is find the sat receiving antennae and put a very fine metallic mesh over it (faraday cage)
all the data is sent via mobile fone gprs so get a mobile fone blocker and situate it close to the unit ..

can't see the problem unless you are doing summit wrong tbh ...