[...] with Moody’s bright-blue eye rotating wildly in every direction, as though to signal to any possible attacker that he was On Guard and Constantly Vigilant
Why does he advertise it? Why not hide it behind an eyepatch, as a secret advantage in a fight?
He does hide it. In the part in the graveyard where he is talking to Snape while they are poisoning Riddle Sr’s grave, he keeps spinning around, despite the fact that the eye lets him see 360 degrees regardless of where it is pointing.
If he’s escorting Potter around, everyone KNOWS he’d be on high alert, so they expect to see the thing whizzing around. Then they expect that when it isn’t whizzing around, it means he can’t see behind him.
And when they try to exploit that, that’s how he catches them.
It’s somewhat like the reason the Dark Mark isn’t always invisible. People will look for a weakness to exploit, so before they find one you should provide them with a fake one.
(If the Dark Mark were invisible and Moody wore an eyepatch, you’d just look for a different avenue of attack.)
You miss the point—he is still hiding it. The eye provides full 360-degree vision at all times, but few people know this, so he maintains an appearance of some level of vigilance.
This means that anyone who sees him respond quickly to a threat (i.e. evidence of vigilance) will have an immediate explanation of how he was able to do so (being vigilant), and not look beyond it to find out the extent of his abilities.
It seems to already be widely known among magical Britain what his noble phantasm is. (Curiously, this didn’t seem to be the case in canon—the Eye doesn’t even have a name there—but Moody is still not sufficiently cautious about it there.)
It wouldn’t have been known if he always had hidden it behind an eyepatch. People need not even suspect that there was anything other than a missing eye.
When Alastor Moody had lost his eye, he had commandeered the services of a most erudite Ravenclaw, Samuel H. Lyall, whom Moody mistrusted slightly less than average because Moody had refrained from reporting him as an unregistered werewolf; and he had paid Lyall to compile a list of every known magical eye, and every known hint to their location.
When Moody had gotten the list back, he hadn’t bothered reading most of it; because at the top of the list was the Eye of Vance, dating back to an era before Hogwarts, and currently in the possession of a powerful Dark Wizard ruling over some tiny forgotten hellhole that wasn’t in Britain or anywhere else he’d have to worry about silly rules.
That was how Alastor Moody had lost his left foot and acquired the Eye of Vance, and how the oppressed souls of Urulat had been liberated for a period of around two weeks before another Dark Wizard moved in on the power vacuum.
He’d considered going after the Left Foot of Vance next, but had decided against it after he realized that would be just what they were expecting.
Apparently he was unable to obtain the Eye of Vance secretly. Besides, the presence of Moody plus the death of a dark wizard known to possess the Eye of Vance plus the missing Eye of Vance is enough to conclude Moody has the Eye.
Why does he advertise it? Why not hide it behind an eyepatch, as a secret advantage in a fight?
He does hide it. In the part in the graveyard where he is talking to Snape while they are poisoning Riddle Sr’s grave, he keeps spinning around, despite the fact that the eye lets him see 360 degrees regardless of where it is pointing.
If he’s escorting Potter around, everyone KNOWS he’d be on high alert, so they expect to see the thing whizzing around. Then they expect that when it isn’t whizzing around, it means he can’t see behind him.
And when they try to exploit that, that’s how he catches them.
It’s somewhat like the reason the Dark Mark isn’t always invisible. People will look for a weakness to exploit, so before they find one you should provide them with a fake one.
(If the Dark Mark were invisible and Moody wore an eyepatch, you’d just look for a different avenue of attack.)
You miss the point—he is still hiding it. The eye provides full 360-degree vision at all times, but few people know this, so he maintains an appearance of some level of vigilance.
This means that anyone who sees him respond quickly to a threat (i.e. evidence of vigilance) will have an immediate explanation of how he was able to do so (being vigilant), and not look beyond it to find out the extent of his abilities.
Quite well thought out, really.
It seems to already be widely known among magical Britain what his noble phantasm is. (Curiously, this didn’t seem to be the case in canon—the Eye doesn’t even have a name there—but Moody is still not sufficiently cautious about it there.)
It wouldn’t have been known if he always had hidden it behind an eyepatch. People need not even suspect that there was anything other than a missing eye.
Apparently he was unable to obtain the Eye of Vance secretly. Besides, the presence of Moody plus the death of a dark wizard known to possess the Eye of Vance plus the missing Eye of Vance is enough to conclude Moody has the Eye.
There’s still a big difference between a very few very knowledgeable wizards hearing about it, and flaunting it everywhere he goes.
I’ve already outlined how a merely knowledgeable wizard can conclude with some degree of certainty that Moody became the owner of the Eye of Vance.
It only required specialist Ravenclaw research to obtain an extensive list of magical eyes. We don’t know how famous the Eye itself is.
What about signalling / intimidation?