A Transmission Called Malice
Nefarious purposes? Not so fast.
Firstly the front end of the receivers in these modern cars are so wide a simple 5W signal anywhere in the vicinity reasonably close to 433.92MHz will de-sense them. There used to be a repeater on the Winter Hill TV transmitter site until everyone who worked there bought a Land Rover, at which point the output at 433.3MHz de-sensed the receivers of their keyless entry systems to the point nobody could unlock their vehicles when the repeater was open. $DEITY forbid they should use the actual key.
Secondly the FM capture effect means someone with a broken, always on keyfob could potentially jam a whole car park if it is sufficiently more powerful than everyone else's.
Thirdly it could just be someone cocking around with a Baofeng (£25) with no malice aforethought apart from showing up these halfwits who are incapable of using a physical key. It may even have been a rent-a-copper's comms blocking the receivers if said car park was attached to one of those fancy, overpriced shopping centres they have around there. Sounds to me like yet another case of herd stupid.
It's highly unlikely anyone would have been deliberately stopping this lot from locking their cars so they could go through the crisp packets, old car park stickers and chocolate bar wrappers in the door pockets or rifle through the Starbucks cups in the passenger floorwell. You'd have to be pretty desperate...