Line your garage with copper screen, like they use in RF labs. That should keep the signal from getting out, and only park in metal parking spaces.
There are plenty of people with the required skills and equipment to do this creepy stuff. There were 800 members in the private detectives' assoc. in Balto. MD, and who knows how many non-members. One ex-cop refused to join the association because he was disgusted with their conduct. He said a convicted felon could get a detective's license in MD.