In case people aren't aware, such a thing _is_ possible. Companies have used steganography techniques in the past to secretly embed identifiers into movies and other visual content. It's been used to track down the movie leakers, for example.

Another example; most printers covertly embed an identifier in their prints.

I have a vague memory of a pre-release video game doing it? Or maybe it was just debugging information that they were embedding. shrug

Personally I don't believe Apple is doing what you describe (though maybe they might do it to a prototype iPhone). But it's certainly your right to hold that belief and take measurements to protect yourself. Shame you're getting downvoted for explaining yourself.

EDIT: Fixed a typo; thank you.