Car locks/unlocks all by itself

My first thought is that there's something wrong with the switch at the driver's lock cylinder (or maybe one of the interior power lock switches- or wiring). If something glitches and commands an unlock, after a time the car will lock (normal if the ecu doesn't see a door open), but the switch is still bad so it unlocks again...

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If your car was wet, then I'd say that was the problem.

I used to think that the best way to break into a Prius was to just spray it with a garden hose, because I noticed it would keep locking and unlocking itself whenever I washed it.

But it turns out that proximity of the key fob was also required. So now I never wash with the key fob in my pocket.

Soap and water running over the door handles can fool the system into thinking somebody's touching the handle.

....Also some Priuses will automatically lock themselves a few minutes after the key fob leaves proximity.

 

Remembering back to my Gen II SKS system, and not clearly, in addition to a sensor on the inside surface of the driver's side door pull handle there is a button on the rear hatch. The integrity of the rubber button cover may be compromised and corrosion may have ensued. The button electrical connections and mounting should be approachable from an access panel on the hatch used to change the license plate illumination bulbs. If you have a fob in your pocket while washing the car, any trickle of water over the driver's door sensor surface will actuate locking/unlocking. Maybe the sensor surface is "shorted" with some kind of detritus. Inspection with a mirror and cleaning might fix it. I never figured out if the sensor on the driver's door was capacitive or Hall Effect, or even infrared, so I'm not sure how the detection is made. The SKS system is designed to time out after a certain time period without fob detection. I noticed with my Gen II that the interior lights would come on, fob detection, at almost 30 feet. Keep your fobs further away from the vehicle or wrap them in a Faraday cage of aluminum foil.

EDIT: Looked at some pictures of the Gen II and it appears that the same sort of pushbutton that is on the back hatch is also on the driver's side door handle. Couldn't remember if there was one. I would suspect the external controls of troubles before anything on the inside, but everything must be checked.

 

key fob proximity auto lock is one of the adjustable options in the base settings. Depending on which level of Prius you have, some are accessible from the main display other require diagnostic tools thru the OBDII connector.
Also check any interior door that has a lock button. I've seen soda spilled into them, causing corrosion and intermittent shorts / activation. They look fine on the surface, but when you pull them out for inspection - you'll see what I mean. Sometimes spraying contact clean down into them will take care of the problem; but once the corrosion has started - it's just a matter of time.

Hope this helps....