Funny story today. I went to Lincoln to get fitted for a new suit (because I want to look my best while driving my Prime). When I was leaving I was backing out of the parking place and my warning beep for a car coming from behind kept going off. I looked around and nothing was there. So I started again and it went off again. Then I noticed the store window. The store was close enough to the traffic on the main road that cars were being reflected off the store window making it look like traffic was moving. I guess it was good enough to fool the sensors into thinking they were real.
Something to keep in mind if considering getting a car with "Auto Pilot." Maybe the sprites, leprechauns, zashiki warashis, gnomes and pixies all decided to leave your house and have a little parade behind your Prime (below bumper level). But just watch out for when the Yeti decides to come check out the car...
Just this morning I had the reverse cross traffic alarm go off and at first I couldn't figure out why -- it turns out that a car was coming close from a perpendicular street and wasn't on my street at all. It's a bit sensitive it seems, but maybe that's a good thing.