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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?

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    All I can think of now is foie gras.. – MKII Jun 2 '16 at 6:54
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    "Voldemort would use the device to find his real enemies and see what they are doing. With it, he would know Harry Potter's real location." No indication whatsoever (that I recall) that that's something you could even do with a Foe-Glass. It seems to just be a rough measure of proximity, he couldn't use it to get Harry's exact location unless Harry happened to be right next to him. – Anthony Grist Jun 2 '16 at 15:54
  • @AnthonyGrist Yeah, thanks for the clarification. I updated the question to take that into account. – RichS Jun 4 '16 at 7:08
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Probably because it was not unique

When Crouch first mentions the Foe-Glass, he refers to it as "my Foe-Glass":

“Oh that’s my Foe-Glass. See them out there, skulking around? I’m not really in trouble until I see the whites of their eyes. That’s when I open my trunk.”

Harry Potter and the Goblet of Fire

Later, when Harry sees it in the Room of Requirement, he refers to it as "a" Foe-Glass":

The walls were lined with wooden bookcases and instead of chairs there were large silk cushions on the floor. A set of shelves at the far end of the room carried a range of instruments such as Sneakoscopes, Secrecy Sensors and a large, cracked Foe-Glass that Harry was sure had hung, the previous year, in the fake Moody's office.

Harry Potter and the Order of the Phoenix

This isn't definitive, but it suggests that the Foe-Glass is not unique: that there may be other Foe-Glasses. As such, Voldemort could probably have procured one himself, and there would be no need for Crouch to tell Voldemort about a device he already knew existed.

As to why Voldemort never procured one, remember his arrogance. He believes that he knows who his friends are. He certainly believes he knows who his enemies are. He also prefers to rely only on himself, for example rejecting the Elixir of Life as a path to immortality. He would have sufficient faith in his considerable powers of Legilimancy that he would disdain the use of artifacts to determine who had lost faith in him.

As an addendum, I am not aware of any indication that the Foe-Glass indicates the precise (or indeed general) location of its user's enemies, merely a vague sense of how close they are. Indeed, it seems likely that it does not even show physical closeness at all, else Crouch would be taking a serious risk letting Harry enter his office without knowing where (at least) Dumbledore, McGonagall, and Snape were. More likely it shows how much danger one is in from one's enemies.

"I’m not really in trouble until I see the whites of their eyes. That’s when I open my trunk.”

Harry Potter and the Goblet of Fire

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    It seems like the sort of thing paranoid Alastor Moody would own, rather than a mainstream "everybody has one" kind of trinket. – Kevin Jun 2 '16 at 6:18
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    @Jonah Too bad the Foe-Glasses don't show up in the stories again. They would make for some nice plot reveals. Hermione could tuck one in her bag when she, Harry, and Ron are on the run. She could take a quick peak at a Foe-Glass now and then to see if any enemies are nearby. – RichS Jun 2 '16 at 6:37
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    @RichS - The stories aren't over. – Adamant Jun 2 '16 at 6:38
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    @Jonah Actually, most wizards would be his enemies. That's exactly the problem. Why would you detect foes if they are everywhere? – Sulthan Jun 2 '16 at 9:38
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    @Sulthan Ah, but the Foe-Glass does not show enemies everywhere, only physically nearby. If Voldy had one, he could use it to reveal enemies within his own ranks. (If he cared to, but apparently he trusted his own skills, and didn't think he needed a Foe-Glass for a reality check on himself.) – RichS Jun 2 '16 at 15:18
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If he did tell his master, I am sure Voldemort would have insisted on keeping the device for himself. Voldemort would use the device to find his real enemies and see what they are doing. With it, he would know Harry Potter's real location. -RichS

Thats not how the Foe-Glass works

The Foe-Glass shows shadows unless you are personally in danger.

‘Oh, that’s my Foe-Glass. See them out there, skulking around? I’m not really in trouble until I see the whites of their eyes. That’s when I open my trunk.’

‘Stupefy!’ There was a blinding flash of red light, and with a great splintering and crashing, the door of Moody’s office was blasted apart – Moody was thrown backwards onto the office floor. Harry, still staring at the place where Moody’s face had been, saw Albus Dumbledore, Professor Snape and Professor McGonagall looking back at him out of the Foe-Glass. He looked around, and saw the three of them standing in the doorway, Dumbledore in front, his wand outstretched.

It took Dumbledore and crew to be actively intending to harm Moody/Crouch, and being within a few feet of him before you could see faces through the Foe Glass.

What use would Voldemort have for something that only shows people's faces when they are already standing in front of him? This isnt some magical mirror in which he can follow his enemy, track their movements, determine who is or isnt loyal. This device only warns you of imminent threats, in your immediate area. Bringing that with you wherever you go is both a hassle and pointless.

Foe-Glass would not help Voldemort in any significant way, as Voldemort did not personally confront enemies until they were captured, or unless he was assured it was safe to attack them personally. Discarding the incident at the Ministry in Order of the Phoenix.

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