Indicator in drivers door

Recently when I lock my Superb the red light in the drivers door stays lit for 30 seconds and then starts to flash. It used to never stay lit before. I checked the manual and it says that this indicates a fault with the central locking or interior monitor. I got my Skoda dealer to check it but they could find no fault and didn't know what was wrong. Any one have any ideas what the problem is?

Thanks.