Several times recently this signal has come on. One time, the dome lights flashed on and off. I stopped the car and re-started. The warning was gone, but came back after a short time. I thought that the seat belt was pressing on the alarm - I had the remote in a pocket. But, today, it happened and after stopping twice to 'reset' the alarm, I kept the key in my hand so that I could see nothing was pressing on it. It happened again anyhow, but went off after perhaps 30 seconds. Perhaps the sensor is going bad? I will call the dealer tomorrow.
Check your lift back door first. This is the most reported cause of this alarm. If you close it slowly you will notice it has two latches. Make sure you close it hard enough that it makes the second latch. If not and you hit a bump you can get the alarm. The fact that the dome light came on one time indicates that one of the door latches was not made.
I've noticed that when there's water on the car (rain or morning dew) that it can trigger the thumb lock on the door when turning. I first noticed this when washing the car with the keys in the pocket, the car will lock constantly. Recently when I was driving, the car would flash "key detected in vehicle" when I was turning. It's nothing serious IMO, just funny that it happens when driving.
Thank you. I had this problem and it was the lift back door as you described. An easy and inexpensive fix I have been a proud Prius owner for a month now. Awesome fuel mileage.
