RF interference can come from so many sources.
Shopping with my wife at Sainsbury's last week, went to unlock the car (2000 Discovery II) and it wouldn't plip, minor panic, tried right up against the roof where the receiver is, no luck. I looked round to see the trolley boy with an electric trolley pusher coming by, waited until he was 20 metres away and the car plipped with no problems at all.
Makes all these Electromagnetic Compatability standards seem like a complete nonsense.
Shaun