From Halopedia, the Halo wiki
Killed Vehicle is a deleted medal which was to be featured in Halo 3. The medal was awarded to players who destroy a vehicle. It is represented by a gray cross with a do not enter sign without text embedded in it. This medal was found in the Halo 3 code, but was not included in the final version of gameplay, and is therefore unachievable.
A similar medal, Vehicle Destroyed, would eventually be introduced in Halo 4 and in Halo 5: Guardians (though in the case of the latter it was added post-launch alongside Score Attack in the Warzone Firefight update).