As Deci has said, this incident seems to have arisen due to use of an out of date map.
The status of many off-road tracks seems to have changed in recent years. Can any regular off-roaders advise about avoiding similar situations in the future?
Also, if you own a cache which is on an off-road track which could be used by motor vehicles, or is likely to be visited by people who might drive on such a track nearby, you might want to check with your local rights of way authority whether or not use of motor vehicles is permitted there, and put a notice on the cache page if it isn't.
Do not go gentle into that good night.
Rage, rage against the dying of the light. (Dylan Thomas)