The greatest weapon of the Assassin class is Presence Containment. When this power is activated, it is possible to completely cut off one's presence. When Hassan is so concealed, it is impossible even for a Servant to sense his presence. One would not be able to tell if Assassin was approaching, if he had already left the area, or if he had even been there in the first place... Thus, Assassin is able to gather intelligence with impunity. Security systems of any kind are meaningless before him.
…
It is hypothesized that the anonymity of being "nobody," the elimination of the self, also influences Presence Containment's ability to "hide oneself."
…
 However, the discussion changes if Assassin uses Presence Containment to obtain a perfect killing opportunity. Even if the first blow fails to defeat the enemy, Assassin can conceal himself again with Presence Containment, and vigilantly await another opportunity.