A little late to the party... but a very good K-9 Officer of mine showed me just what can happen with the readings the on-dash radar units used in a lot of the cars.
It was a chilly November in Oklahoma and he and I were chatting while he was on his Lunch break... I was standing outside the car and noticed on the dash, his radar unit kept picking up speeds upwards to 40mph... he was parked in front of my house, a heavily populated residential setting on a Military base. Because the car was facing a curve in the neighborhood, I asked him how he could be picking up radar hits when there wasn't any traffic and he was pointed directly at the house in the bend of the curve... oh, did I tell you it was midnight?
Anyway, he leaned over and turned up the Fan on the defrost... the radar unit then started getting 70 and 80mph hits, with the defrost on medium... and a little over 100mph with the fan on high.
Not saying that anyone out there is doing this... but with all electronic devices, there are always some bugs in them that simply can't get worked out, and for the most part don't need to be.