The next morning (in the shower, of course) I had a great conspiracy theory thought. Modern ink cartridges are more than just ink reservoirs, they have the print head built in. And they have some basic intelligence to tell the printer their ink level, type of cartridge, etc. Some have had crypto chips designed to make it hard for 3rd parties to offer compatible replacements.
So what if each cartridge contained flash memory and recorded everything it was told to print. Or maybe just documents that contained keywords (it could have a rolling buffer and only store documents that match some list). It wouldn't even have to record the text per se, just the print commands - that's easily decoded back to text since you just re-run the sequence to 're-print' the same thing. Offering free recycling gets your marks to do the work of sending the intelligence back to you. And remember, in the past typewriter and printer ribbons have been used, many times, as sources of intelligence since you can often see the letters that have been struck on the used ribbon. And there are known tricks from the cold war including using Xerox repairmen to plan cameras in foreign embassy copy machines so that every document copied was also photographed.
Yes, it is totally out there and, no, i don't believe this is happening. But just a little bit of paranoia for those wary of the panopticon. Doing something like this gets easier all the time, think of just how small microSD cards are.
And now that's going to be in your brain.