Could be the car key. After a few years they tend to pick up natural oil from your fingers , which combined with dust etc can make the buttons stick. It may be that after pressing the lock button it doesnt pop out fully and remains perilously close to transmitting further VHF locking signals when the key moves , or temperature changes etc.
If you have a spare key try using that , keeping the original key out of range of the car.
If this solves the problem you may be able to clean up the original key buttons with some electrical contact spray, or sparingly use some other cleaner with an old toothbrush etc. You might be able to do a better job, with less risk to electronics by opening the key as when changing the battery . But I wouldnt risk cleaning unless you have a spare key available.
