Every piece of mail that passes through a high-speed sorting machine is scanned, front and back, OCR'd, and stored - as far as we know, indefinitely. That's how they deliver the "what's coming in your mailbox" images you can sign up to receive via email.

Those images very often show the contents of the envelope clearly enough to recognize and even read the contents, which I'm quite positive isn't an accident.

The USPS is literally reading and storing at least part of nearly every letter mailed in the United States.

The USPS inspectors have a long history of being used as a morality enforcement agency, so yes, this should be of concern.