My husband and I had a close call with one in the grocery store parking lot. He had just opened the car door when the Prius parked next to us just took off without a glance in our direction, coming close to losing its mirror and getting the side scraped up. We had no clue from her movements in the car that she was about to "go." Then the woman turns down the wrong direction on a one-way lane.
There's also a silver/greenish Prius that I see every so often on my route home. The woman hates me for no reason. I can't wait to run into her again at a light so when she rolls down her window to yell at me I can find out what her problem is.
And yes, they are silent at slow speeds which is dangerous in parking lots, cross-streets, over-taking, and side-street pullouts. I tend to use all mysenses when riding, including my ears ("that sounds like an 18-wheeler with a trailer/without a trailer...a delivery truck, a car, a motorcycle, a scooter, another bicycle - most have some rattle/braking/gear noise/rider noise...but slow moving prius?"