An article from the Straight Dope mentioned by Green in the afterword:
Do maps have "copyright traps" to permit detection of unauthorized copies?
August 16, 1991
Is it true that, as my father says, companies that produced maps (Rand McNally, etc.) make up some little bitty towns and dot them around their map design so they can tell if anyone copies it? Has anyone ever gotten lost trying to find one of those made-up towns?
— Susan Owen, College Station, Texas
You are talking about "copyright traps." They are devious. They exist. In a world of high-level conspiracies that are completely imaginary, it's a relief to discover one that's not.
For the record, the folks at Rand McNally swear on a stack of road atlases that they would never use copyright traps. However, they admit a small regional map company called Champion they bought a while back did put a copyright trap into a map on at least one occasion. The trap consisted of a nonexistent street stuck into a map of a medium-sized city in New York state — a fact that was gleefully revealed on a network news show.
On investigating, Rand McNally found some smart-aleck cartographer (and you know what a wild and crazy bunch they are) had gone ahead and done the wicked deed on his own. Whether the guy committed other cartographic sabotage I don't know. But the possibility of additional fakery does exist — and may for a while, since checking every detail of a map is a huge job. Not that I'd get into a panic about it, but on your next road trip you might want to bring a flashlight just in case.