Wow, this is exactly the same fault I had on my R reg Escort. The alarm would wait about ten minutes after locking up with the key, then would start blasting for no apparent reason. In the end in desperation I disconnected the alarm noisemaker which is found at the left side of the boot under the side liner. This stopped the neighbours being upset, but of course it didn’t stop the lights from flashing. After trying several different ways to lock and unlock the car without making any difference, I struck gold. It occurred to me that there must be some residual power in a capacitor that was discharging after the ten minutes lapse, so I tried unlocking the car with the key by turning the key to the right and HOLDING IT AT THE TURNED POSITION FOR FIVE SECONDS before taking the key out. If you want, you can look at the red alarm light on the dashboard, and when it goes from steady on to blinking again, whatever it is that causes the fault is put out of action. I’ve locked the door using this method hundreds of times since and not once has it caused the lights to flash after locking.
Don, May 2009