There is a tendency, when becoming acclimated to the idea that one can use any Score (Stamina, Will, Lore, Cover, even Humanity) for a "Perception Check" related to noticing something, to consider how such a thing would apply to that classic of RPG gaming, the "Surprise Check".

Example

A demon has a sorcerer pinned and announces to strike a finishing blow. That sorcerer's demon announces to explode through the wall (surprise!) and bowl his master's assassin over.

How do you work the "Surprise check" into the system?

Frankly, you don't. Everyone announces their actions for the turn, just like normal. The sorcerer's demon gets a couple of bonus dice for tactics.

That's it. No problem, no extra rolls, no "perception checks," none of that stuff.

When would a Perception check vs. some surprise action be combat timed?

It wouldn't. If, after the Announcement Phase and rolling dice for you actions, the person doing the 'surprise' acts before you, then it's likely (depending on narration) that you were surprised. If you beat them, you weren't.

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