I like to run the game "realistically" when it comes to such things. Passive perception is just that, passive. For someone like Pyre, there is a huge difference due to disadvantage. With just darkvision and seeing in dim light, and with sound muffled by distance, barriers, and possibly other things, Pyre's Passive Perception is 19 instead of 24. Now, if Pyre is
actively perceiving (via her action or some other feature), due to reliable talent, even with disadvantage she can't get below at 24. Unfortunately in Pyre's case, 19 wasn't good enough, but 24 would have been...
Anyway, in the case of Dread Counterspell, Venca just utters a word and the target takes psychic damage. I don't see any reason narratively while if he is successfully hidden such a feature would reveal his location, especially when opponents are not looking for him even in the vicinity where he is.
Finally, although a technical issue, the fact is Dread Counterspell
isn't a spell. It is a feature, not an attack, even though it deals damage. I see it as rather an indirect thing since it requires no attack roll nor does it use his action, just a reaction. Otherwise, what can I say--it's Vecna.
