WFRP 3e makes the classic mistake of many RPGs in its skill system... it addresses in good detail how players use their characters' skills actively when it is their turn to act, but it doesn't offer much at all about passive skill use. But, the two adventures I have seen-- A Day Late and An Eye for an Eye in the Tome of Adventure --both rely on passive checks.
Anytime I pick up a new RPG, I do a really good job of reading it with a newbie's perspective. Through that lens, the only way the book says I can use a skill is by performing the Use a Skill manoeuvre when I am the active player . But, being the seasoned RPG gamer and developer that I am, I infer that the GM can call for a character to make a passive check anytime. During story mode, this is no big deal. But what about during Encounter mode?
Using an example from A Day Late , let's say a character engages a beastmen engaged with the coach on his turn. The scenario states that the character can attempt a Hard Observation Check to hear the person inside. Obviously, the player will have no idea to do this check unless the GM tells him to. But is the intent for GM to force the character to use a manoeuvre to do the Observation check? I hope not, and I don't think so. I think it's a free passive skill check.
Basically, I think any scenario input that calls for a skill check that the characters don't really realize they are doing is a passive skill check that is done 'for free.' But it would be nice to have it written somewhere how WFRP expects GMs to handle passive skill check situations.
Thanks!
Steve