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Post Office executive denies cover-up over Horizon back door
(www.bbc.co.uk)
This is a most excellent place for technology news and articles.
Probably because some n-eyes (is n still 5?) nation state "we're the good guys, honest" protocol insisted that there be one. And if it wasn't that it was probably some misguided attempt to permit remote maintenance.
The former hiding as the latter is not impossible either.
My guess is it’s much simpler: some genius at Fujitsu thought it would be helpful if they could remotely log in to machines for support reasons. They (unreasonably) didn’t want to send people onsite and they (reasonably) didn’t think Post Office workers would be able to resolve issues over the phone.