I am a recently retired carrier. Your mail is being delivered by a "Utility carrier" This person is the sub on 5 routes,done in rotation.
In the case of an unscheduled absence it is more than likely that your route has been broken into sections, and given to carriers that were deemed to have, "downtime" on their own route.
Another practice that has been put into place is to keep several vacant routes. These are filled by transitional non career employees that are used as needed.
True the case does have blue blockers in it so hold mail is not cased, however, 90% of the letter sized mail comes presorted as Delivery Point Sequenced mail. This mail is taken to the street just as received. In fact if caught even fingering through this mail while on office time is grounds for discipline!
Your regular guy and the Utility if worth their salt, will see your mail, and not deliver it. A transitional or downtime carrier will have no way of knowing that you are on hold.
There is no excuse for unsealing a by any one but your regular. When I came back after a day off I made it a practice to check every boz on the route.
Not that any of this fi+es the problem or makes you feel better,but I think it does explain why it happens. Of course ineptness and lack of caring should never be ruled out either.