Whoever told you to leave the key in the car, when you get out of the car is a fool and is very bad infomation. Would you leave the key in the ignition of a car without keyless entry or start etc. What would the insurance make of it, if it was then stolen.
As many have suggested you can use the key hidden inside.
Some cars do lock themselves, if you say open any of the passenger doors or the boot and then close it and do not open drivers door and put key in and turn on the ignition. Many years ago this caught me out and had to call green flag and had the same done to the door with an air bag, should bend back easy enough, It was raining stopped to get a coffee thought, I’ll give wing mirrors a wipe, so put coffee, wallet and key down on passenger seat, cleaned window closed door was walking round and i hear it close, oops.
Now i never ever leave the key in the car.
Maybe it’s a faulty sensor or like with my old vw a programing error that happens when certain perimeters are set or met, maybe the system crashes and the default setting is locked.
My golf would not move away sometimes if you had acc on and auto hold at same time only happened at traffic lights with no car infront, so you had to brake to stop, had some update to cars software when it was in for service and it never happened again.
Only way to reset it was turn car off then restart, was crazy. Vw could never help as it never happened to them, when they had the car. So i get how you feel, I think you should contact motability especailly if they cannot fix this problem and tell them what you was told by seat.
Personally i don’t want a car with keyless entry, but it is becoming the norm. More things to go wrong. Central locking works fine and IMO is more secure.
I always lock my car when getting fuel as i seen videos of people sneaking up to the passenger door opening the car and stealing stuff, like a hand bag or a phone and we are not covered for anything left in the car.