Wow!!! Your dad has obviously been retired for a long time. We have had our mail diverted and stolen twice now for weeks at at time after people filled out fraudulent change of address forms. Do you want to take a wild guess at how “seriously” the Post Office took that? It took multiple phone calls, letters and emails to get it straightened out each time. We were never contacted by any postal inspector and they would not even tell us where our mail was being delivered to.
My step grandfather worked for the Postal Service for decades... it is not the same these days.
A postmark is one of the very easiest “legal government acts” to forge. No apperant effort has been made to make them difficult to forge. The link in the post you are responding to will make you a Postmark stamp to any specification that you want, with any font and design you desire for $15.95. It is not even a crime to possess such a device or have one made. And no the post office couldn't give a rat's behind if you are not using it with criminal intent. But even if you are... they would likely not be any more interested than the FBI was in Hunter's Laptop.
The postal inspector has no need to talk to you.
If you look at each piece of first class mail, there is a bar code on the bottom. That comes when they take a photo of each piece of delivered mail.
They simply track those scans backwards.
Every time your mail moves from one place to another ther is a picture. If the mail was redirected anywhere before your carrier, they know where.
No, I am not behind the times. I think you are... thinking they would have a need to speak to you. You would literally have nothing to add to an investigation—except consuming an hour of their time. And based on the way you write, it would be a waste of everyone’s time.