To get the super secret stealth bike you need to go the Mangled
Memories level in Rocket War and drive straight forward until you get
near the wall.
Bail off your rocket before you hit the wall and run clockwise around
the arena agents the wall. As you run to the right along the wall you
will pass several bends in the wall, at about the third bind there is
a hidden hallway that you can walk into. You have to be on foot
because you can't drive a rocket through the wall. You have to be
running to the right because the opening if very small and you may
not see it running to the left.
Once you go down the hallway you will see a rocket inside a fenced in
area. Get your new rocket and win the war. You must win the game on
that rocket for it to be one of your rocket selections.
The special thing about this rocket is that it can not be seen on the
radar. You have to careful not to lose you rocket to another rider
because you wont be able to see it on your radar and it will be hard
to find.