h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A report has revealed that two traffic patrol officers from North
Berwick were involved in an unusual incident whilst checking for
speeding motorists on the A1 road between Oldhamstocks and
Grantshouse. Last May, they were using a hand-held radar device to
trap unwary motorists on the Edinburgh to London trunk road. One of
the unnamed officers used the device to check the speed of an
approaching vehicle, and was surprised to find that his target had
registered a speed in excess of 300 miles per hour. The £5000 machine
then seized up and could not be re-set by the bemused PC` s.

The radar had in fact latched on to a NATO Tornado aircraft in the
North Sea, which was taking part in a simulated low-flying exercise
over the Borders and Southern Scotland. Following a complaint by Sir
William Sutherland, Chief Constable of the Lothian & Borders Police
force to the RAF liaison office, it was revealed that the officers had
a lucky escape - the tactical computer on board the aircraft not only
detected and jammed the "hostile" radar equipment, but had
automatically armed a Sidewinder air-to-ground missile ready to
neutralise the perceived threat. Luckily the Dutch pilot was alerted
to the missile status and was able to override the automatic
protection system before the missile launched.

The Police have so far declined to comment, although it is understood
that officers will be advised to point their radar guns inland in future.

Good story, I will include it in the final piece. I'll have to change the missile type to a HARM or Maverick, though, since the Sidewinder is in fact an air-to-air missile.

This story also appeared in New Scientist. A few differences, British Pilot, Harrier Jet on training manoeuvres, unarmed so no need for pilot to intervene. If there was an original true story it has now become altered and embellished and difficult to trace.