My advice is that every member of staff should have access to the building and there should be someone responsible for locking up that building who doesn't go home until everyone's card/PDA/van is back or accounted for.
I've been stuck in the RLE until nearly 4pm on a Saturday before now because someone was late back. I made sure I got paid and some extra for it but the point was that everyone was accounted for.
I would have thought there was a duty of care to a) ensure the staff have actually finished their duty, in other words, they're not lying dead in a gutter somewhere and therefore still RM's responsibility and b) ensure all the mail has been delivered or is accounted for.
There shouldn't be a scenario where offices are being locked while people are still out on delivery without prior arrangements being made if necessary. It's wrong on so many levels. I'm sure it should be a Health and Safety (welfare of the staff) issue and well as a safeguarding the mail issue.