Most Wanted

Most Wanted is an unmarked side quest $$ As of patch 1.4.0.5xx, it is now a component of the larger Faction Hit-Squad Master Quest, along with the anti-Legion quest Caesar's Foe.

Detailed walkthrough
Should the Courier's negative reputation with NCR reach a certain point, this hit squad will be dispatched to kill them.

A "warning" squad is dispatched within hours of the Courier first attaining a sufficiently negative status with the NCR, warning them that if they do not improve their standing they will be treated as an enemy. Thereafter, NCR Ranger hit squads are dispatched approximately every three days, depending on whether or not the Courier is located in an area where they can be attacked.