This pronouncement of hers has the same effect on Anakin dumping a bucket of ice water on him might. He stares in shock, forgets to keep up his illusion of other-hims and now there is one bright presence in the Force, shining directly behind her.

Of course, the reaction that always immediately follows shock when one is dunked in such a way is annoyance. And in Anakin's case, more than annoyance as well as a grim determination that she will not have Obi-Wan. And since his cover is, essentially, blown, he ignites his lightsaber. The snap hiss seems unnaturally loud in this desolate area, but he doesn't care.

"You cannot have him." He even manages to say it evenly!