Ghost motorcycles are normal freeways or other countryside-style bikes that spawn with no driver but are usually reported to be turned on.
They spawn like normal bikes, in normal places, but are not being driven, which makes them even more confusing to players who are unaware. This tends to cause monumental accidents, and this could even lead to shoot-outs in some occasions.
There are also reports of them riding by themselves.
This is most likely a glitch, where the bike spawns, but the game doesn't spawn an NPC fast enough, or the one it chose was corrupted, does not exist, or is from the wrong place (e.g. Spawning a Beach Bum on a Dirt Bike.) This is why sometimes the bikes are reported to move by themselves.
They can be most easily found on the highway surrounding Back O' Beyond.