SH4 works by a virtual ship system. If ships are a certain range from you, within sensor range, they are generated and plotted as single ships. If they are outside whatever the critical range is, they are virtual ships: SH4 knows there's a convoy there, but doesn't actually plot it. This saves memory, because the whole ocean scratchpad is just notes, not actual ship identities and positions.
Things get trick when ships cross that barrier. Of course, when a virtual convoy enters the critical distance, the phantom ships are generated, identified and plotted to become real ships. But if real ships cross the boundary, they will become virtual, perhaps not to be replotted ever again! (Play the Twilight Zone music!)
Luckily, under most circumstances this happens like a magician switches cards while you are distracted. Unfortunately you're a sharp-eyed SOB and caught the ole switcheroo. Now you're not buying the trick and you're telling your friends the magician is a phoney.
