I also believe that all mail is photographed during processing which would reveal 1) When it was post stamped, 2) where it was post-stamped, 3) if it was post-stamped at all (ie rather than delivered in a wheel barrel through the back door).
*********
Currently postal patrons are sent photos of their incoming mail a day or two out from delivery, so this is very probable.
I get an email with photos of my mail every morning. I’d say that it’s about 95% good - misses one once in a while, with a drawing of an envelope with the note that that particular piece of mail can’t be identified.