> Seemingly random pixel positions shift, brightness shift, density shift, etc.

These are all hypothetical though. We don't know if these techniques exist in the wild.

I did a quick study on an Epson Ecotank printer 3 or 4 years ago, and I found "random" colored pixels that were added (for example a single cyan dot in a large circle of pure cyan changed to a pure magenta dot... like that is a natural mistake if it occurs multiple times on a single page with different colors, and different positions), fuzzed black lines (not really fuzz, but holding data in the "noise"), and a few horizontal lines shifted by a few pixels every once in a while. So I believe that most of the other changes are real.

Edit: If you want to check your printer then: print something to a piece of really white paper, scan it, and then use a diff tool like in Gimp after resizing it to easily spot the differences. If you are using off white paper you can scan it also before you print on it, and attempt to remove the paper's natural color variations before "diff"ing it with the digital version.