Ghost Motorcycles are Freeways or other countryside-area bikes that spawn with no driver but still have the engine running.
They spawn like normal bikes, in normal places, but are not being driven, which makes them even more confusing to players who are unaware. The bikes tend to cause 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. 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.