Altaïr is sure to keep himself subtly hidden from view of the street most of the time. High up enough, and he doesn't have to worry about even that.

He did not account for extremely keen, inhuman eyesight.

It is as he's resting that he spies someone following him. He gives a small start, but hardly perceptible to himself because it would be a bad idea to fall from mere surprise. He does not court these heights without knowing what he is doing. He thinks to make his escape--climb higher and find a suitable place to jump into a haystack--until he recognizes the dark shape of that coat scaling the tower in lizard fashion. The jig is up anyway.

"What do you want?" he demands.