I think that your aspects should only be compelled when that cause complications for you.
So the knight just happening to walk by the crime scene is the sort of thing that happens to knights all the time. But you could compel knight of the cross if the knight had something else he needed to be doing at the time.
Something like:
Traffic is very bad today, you are running late for work/date/important meeting. When you get to the blockage, you see (the crime scene).
And the knight can take the compel to stop and investigate.
Or something that the knights do all the time causes him problems, something like:
You are attacked by (minor threat) in an alley. You are chasing it down when you come running up to (crime scene). The police look from the (chopped up body) to the crazy man with the sword and you can almost hear the gears in their heads turning.
And the knight could refuse the compel by, say, declaring that he had helped one of the cops on the scene in the past, who convinces the other cops there that he is just a harmless eccentric with a sword, and not the mass murderer they are looking for.