If you're driving a vehicle at the time then the killer car will simply stalk your movements, and will eventually go away if you continue driving around without engaging it. If all of those points have been met, then the GTA Online Phantom Car may spawn near you at some point – it looks like the Declasse Tornado Custom pictured above, and although it doesn't appear on the minimap you will hear an ominous revving sound when it shows up.
You need to be in Free Roam mode and not inside a building.
