[muawiyah]

It would take entirely too much time to discuss all the things you can do with encoded invisible processing information ~ but one example ought to help. Let's say the visible cancellation says Los Angeles, and the return address says Los Angeles, but the first point of barcode application is actually New York City ~ that barcode information will be invisible.

There are some normal events that could bring this about, but there's also the possibility that somebody in New York is entering mail pieces that you'll think are from Los Angeles.

The contents may well have no meaning at all until you, in Philadelphia, read them thinking the mailpiece originated in Los Angeles ~

Could be a mad bomber at work; perhaps someone sending poisons to you; possibly someone commiting some other sort of crime against you ~

It is possible to run mail covers on everything these days ~ and with just a bit of operating information you can probably surface suspicious mail and send it to the postal facility that does chemical analysis on it first before it's sent on to the customer.

[loungitude]

Thanks, but the invisible markings are readable by the post office right? Are you proposing a conspiracy involving the PO?
Not that I think it’s improbable, ...