Seconded!
I was hunting ghost S/A contacts on various Multiplayer Servers yesterday (Buddyspike & 104th mostly).
Three or four times I was hunting an S/A contact that stubbornly refused to show up on my Radar, all the way till he was supposed to be a mile-ahead and on my altitude. But there was simply nothing.
You can usually tell by looking at the altitude and speed indication. It remained stable for several minutes.
Cheers,
FortunateSon