As far as I know, a problem with VC's map/engine - doesn't always happen, only sometimes when you drive to far away from the vehicle. The vehicle may just disappear altogether (what is happening is it sinks into the ground [I think]), or, as in your case, it will then respawn on the nearest car-path (ie. in the middle of the road nearby). Using the remember car feature of my scm, or using Spoosh to spawn the car, will make it so its almost always the latter of those two possibilities, but I don't know of a way to stop it from happening in the first place (although Hay's suggestions sound like they're worth trying).