Wellll, hold on there, speedy. The principle that makes this rule good for those people with player-characters may hold for the GM too.
So, first and minor point is, sure - if an NPC is doing a ritual, then it's not like lightning will get you if you have the ritual's effects hit the situation from off-screen, as it were. But ... I suggest not doing that.
The second, primary point is that Circle of Hands is all about the immediacy of intentions. If one of my NPCs were doing a ritual, then there is no way that a bunch of preparatory events and a bunch of during-event phenomena would be intrinsically invisible to the player-characters. Wizards, in particular, have senses that tune into such significant actions, within the whole range of the venture. Less direct means apply as well, in that beginning such an activity changes relationships, changes the circumstances, changes the balance of social power. People respond to that and react strongly with goals of their own, and Circle knights, especially if they're from the region and/or the relevant social ranks, see those responses as clearly as anyone.
Don't think of these as clues or hooks. Think of them instead as nothing happening in isolation in the venture, ever. The wizard in question has relatives, acquaintances, and local history, right? (Rhetorical question. He or she must.) They are invested in the ritual too, in any way you can think of.
The third, supporting point is that we're not talking about back-story in this post at all. If a ritual is part of the back-story, then fine, it is, or rather, it was, effects and all. But if you're going to say this, then be sure the ritual is over and all that's happening is aftermath.
OK - so how, then? Basically, you're starting a scene which includes the wizard engaged in the ritual, as well as Circle knights who are currently not perceiving that this is so. The only issue, then, is to go right ahead and start playing those hours during which the ritual is under way. The wizard is doing. The Circle knights are doing ... what? Whatever the players say, based on what the knights see and do. What happens?
I think you'll find the ritual doesn't stay off-stage for long.