The Gift isn't about telling him where things are, though.
The main utility Masks would get from it is the fact that it leaves a "blank" spot for him to notice, an area that he suddenly won't know anything about.
The divination itself isn't as important when compared to the ability to notice when a "blank" shows up on it, indicating an Anchor, Noble or Excrucian-shard.
Even if you act with stealth, you will still leave a blank spot, won't it?
That is, after all, a major part of miraculous beings - they leave blank spots on Divinations.
Otherwise, it'd be easy to find, say, Cneph or find out who gave the Excrucians their first Estate, since they wouldn't leave blanks on Divinations.
In fact, even if they use a miracle, to hide themselves in order to sneak up on him, that also leaves a "blank spot" for him to notice, doesn't it?
And, since nothing short of Imperial Miracles (and Blind Lie, I guess) can alter Masks perceptions or thoughts, he should notice when an area suddenly appears as something he can't know anything about at the moment.