Scenario: Britannia has grown tired of the terrorist known as "Zero" reeking havoc on their forces and wants to take him down once and for all. They know that if they figure out who the man behind the mask is, then they can catch him, so they hire the world's best detective, "L" to solve the mystery...
-L gets all info on Zero that the Britannian family knows, he's a brilliant strategist, he leads the Black Knights, he's fighting to overthrow the Empire, etc.
-L knows that Zero operates in Area 11 and made his first appearance in the Shinjuku Ghetto area.
-Lelouch doesn't know that L is after him, unless L decides to state it publically.