Sounds like a neighbour has one of these:

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GPSTracker Defence
April 21st, 2008 by Matthew in GPS gadgets 
If you are the paranoid type who worry that someone always has their
eye on you then this Anti
GPS Tracker is for you. What this anti
tracker does is block all
GPS signals for a 5 meter radius making it impossible for a
tracker to track where you are and send those details on to others. The Anti
GPS tracker plugs in to your
cigar lighter and is powered from the 12V supply.
One thing to note is that by blocking those “tracking” you, there will be the problem that your
navigation device will also not work… so you really need to weigh up your paranoia with being practical here. It still is a fairly cool
gadget though and could keep you protected should you find your self in a situation like that.
The Anti
GPS Tracker costs £149 and is available at the
link below.
Product Page - Source
SpyReview