Can your employer fit a tracker to the vehicle that you use for work without telling you? Would your rights be affected if, say you stopped and bought a newspaper/ sandwich in work time?
If you were being tracked because your boss had good reason to suspect you of gross negligence or criminal activity, he does not have to inform you that you are being tracked and can use it as evidence
I think a tracker tracks your route, not what you are buying/eating. As long as the sandwich shop is pretty close to a reasonable route to where you are going there shouldn't be a problem.
What reason has the employer given for introducing a tracker?
And what is your concern?
If it's their vehicle they can have the music from an ice cream van and a large stuffed budgie attached if they want.
Their rights would be affected if you were skiving, in that you use time they're paying for to do your own private things, in exactly the same way as a factory worker would expect to be seen if they popped to the shops.
That said, if there's no abuse I can't see any reasonable employer objecting if you stopped to buy your lunch.