I have known this across many makes and models of car.
The first (and most common) possibility is that something has held the unlock button on the fob.
Disable the feature, and test it out by holding down the unlock button.
If this is truly disabled - the second probability is RFI related (if the dealer has discounted an electrical fault with the car).
Something in the vicinity is radiating a 433MHz (UK) frequency, or an IF of that frequency, causing the car to unlock. It's a ****** to find, as it could be something like next doors microwave oven, garage door opener, remote thermometer, remote control gates, wireless alarm devices, remote meters, etc.
I successfully demonstrated this on my neighbours brand new 5-series BMW. I brought home a lowish power RF link from work, pointed it at his car, and fired it up. All his electric windows lowered, doors unlocked, hazards flashed, and his instrument pack lit up like a christmas tree!
If you live around Droitwich in Worcestershire, BBC R4 LW at 198kHz pumping out 500kW of RF power usually locks people out of their cars, swamping the mW's that the remote fob the car uses!
