I never said a mage would know that it happen. I said there would be proof that it happen via a weak veal, that can be sensed by the mage and malevolent spirits around. And it would be spoken off. Everyplace that had something like that shows it in it's environment and the weakness in the veil. Redcliff my not have the environment totally changed by it but the veil is still weak there. Added weaken veils can because by malevolent magic alone so say just because the veil is weak does not mean it was caused by malevolent magic. Just because basin has rifts does not mean it case be harmful magic. With redcliff it's the case because we know the history there.
And the Avvar don't hide thing that happen. It's not in their nature. Sorry, but it's unnatural to there culture to hide info benevolent or malevolent. Their culture is too simple for that. So no they would not hide it.
I don't remember ever having the relatively weak Veil (I say "relatively weak" because the Veil as a whole is being fed through a paper-shredder at that point in the game) mentioned during In Hushed Whispers, despite logically knowing it should be there. Might be because I didn't bring Solas, since I switch pretty freely between my mages and don't remember who I brought where except that I hadn't met Dorian yet. Still, if you were right that we would have heard about a relatively weak Veil in the Frostbacks, I'd think I'd remember hearing about it in Redcliffe. Although you're right that this entire line of questioning is probably pointless because spirits coming and going is in and of itself a feature of the Frostbacks rather than a bug.
Do you ever have the option to ask if their system is flawless? I'm still not sure I'd trust them, since "it's unnatural to there culture to hide info benevolent or malevolent" doesn't seem to be a strong enough guarantee that I'd trust someone from this setting implicitly. (Especially since I think I remember coming across mention of an abomination who A: violated their cultural norms regarding spirits and B: tried to hide it. But even without that, this isn't Faerun. It's Thedas.) Still, unless you explicitly ask, you might not hear about it because it's so rare as to not be worth mentioning. This might be encouraging, but it's not what you said the case is.