It's not faulty wiring.
They have movement detectors that were supposed to detect the presence of a person that had become "locked in" and thus alert the driver.
As I understand it, the alert for the driver was disabled very quickly given the rate of false alarms caused by wobbling yorkies (container mailed is carried in).
As you'll have noted, given the large number of blue lights lit and seemingly random pattern between coaches, the system has a very high false positive rate and is now completely ignored...
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As an aside, there was a planning application at Peterborough Mail Centre (Papyrus Road) back in the early 90s to add a spur off the ECML and platform (with cover) for the reception of mail trains. Presumably the 325s. It would have been parallel to a service road within the Royal Mail depot.