If it’s a single player game or you have admin powers on server, try going to the marker spot and using god mode go below and above the map to see if the vehicle is either really high up or far below map surface. If that can’t be done or you still can’t find the vehicle then I believe if you place a different vehicle of the same type, it should go away. AFAIK, the game only remembers the marker for the most recently placed vehicle for each vehicle type. So if you lost a minibike, put a second minibike down and you should only see the new minibike marker. Hope this helps.