Barty Crouch Jr., while impersonating Alastor "Mad-Eye" Moody, had Moody's Foe-Glass in his office during Harry Potter's fourth year at Hogwarts. Near the end of the book, Harry Potter and the Goblet of Fire , the Foe-Glass shows Albus Dumbledore, Minerva McGonagall, and Severus Snape just before they burst into the room to arrest Barty Crouch Jr. Barty Jr. had an entire year to learn about the Foe-Glass. He could have told Voldemort on any day that year about the Foe-Glass. If he did tell his master, I am sure Voldemort would have insisted on keeping the device for himself. Voldemort could use the device to find out if any Death Eaters near him are really enemies. Although the harrypotter.wikia.com page says the Foe-Glass is a dark detector, we know from this question and from the book itself that it shows the foes of whomever physically possesses it. There is no indication he ever told Voldemort about it. Why not? Answer Probably because it was not unique When ...