I've also noticed some people vanish when you start approaching them in SC, if they don't leave a high/low energy wake then they saw you coming and logged.
Why they can't just take evasive manoeuvres at that stage is beyond me.
that one is a zoning problem caused by flying around in supercruise, you can never see everything in a system and some things pop in and out as you receive data for them, if somebody disappears right in front of you chances are they will reappear on the same path further down. For example yembo had two "zones" i'll call them, the station side, and the sun side with about 1000l's in between, if you were at the sun you couldn't see people at the station and the reverse. If you flew chasing somebody from the sun they would disappear when they got to the station side and you were still in the sun side, but then transition back out when you also entered the sun side.
Bit of a vague explanation it works sort of like that, but it does mean targets pop in and out a great deal.