On our 2015 Prius V, suddenly the dash light showing the door was open wouldn't turn off. It kept blinking, and that beeping sound kept going whenever the engine was one. We tried opening and closing all the doors, maybe 10 times. I tried pushing the little switch plunger sensors in manually. I tried turning the car on and off. Nothing. Nothing. Nothing.
Called our local mechanic, and they referred us to a body shop. Got the to the body shop, told the owner what I'd tried, and he told me had a nearly identical Prius in his shop with the same problem-and he'd tried taking the sensors apart, and with no luck. I told him what I'd tried, and he generously walked around the car with me, to see if we could figure anything out.
Turns out that there is a foam rubber gasket around the rear hatch, maybe 2" x 0.75". That gasket had popped by an inch or two. While it would still compress enough to allow the rear hatch to close, it pushed out the hatch just enough to keep the sensor from knowing the hatch was closed. The owner spent maybe 3 minutes pushing the gasket back into place with his fingers, and....POOF! MAGIC!. The problem was gone.
He wouldn't take any money-told me that I might have helped him with the other Prius in his shop. I'm going to go back tomorrow to have him do some other minor work-he's earned the business.