[RobW]

Short answer -- I've typed and deleted a bunch of answers, really not sure!

Let's go with the idea that the illusion reflects the beliefs of the observers. If the observers think the illusion should know about something, then the illusion would act in that way, and otherwise not.

Scenario 1. Two figures: (1) a wizard creating a wolf illusion; (2) a thief who, unknown to the wizard, is hidden behind the curtain. The thief can see the wizard and the illusion. No one in this case has a belief that the wolf should see the thief, and so the illusion doesn't.

Scenario 2. Same two figures and conditions, but this time the illusion is of a bloodhound. The thief might believe the bloodhound COULD sniff out her presence. Maybe it does...?

Scenario 3. Same as scenario 1, but the wizard directs the wolf to poke its head behind the curtain. The thief now believes that the wolf can see her, and so it does.

Scenario 4. Similar to scenario 3, except this time, for whatever reason, the thief can't sense the wizard. Now the thief has no belief for or against the illusion, and the thief's beliefs aren't part of the illusion's "knot of force". In this case the illusion can't take into account the thief's beliefs. This is like sending an illusory bird over the castle wall. The people behind the wall have no beliefs about the illusion and so the illusion doesn't see behind the wall.