I think the point was that it's difficult to notice in the first place, not that it would be hard to find once you know you're looking for something. You don't have a black WiFi router with antennae dangling down from the ceiling.

If you went the other direction and didn't worry about it being noticeable, it would be kind of a fun project to break up a book into a series of QR codes. A scavenger hunt, with each code's text ending with a clue of where to find the next?