Did Malaysian Airlines 370 disappear using SIA68/SQ68 (another 777)?
I think Keith is on the right track (excuse the pun).1. The weather. Check on that. On a clear night it is possible to sneak up behind and tail an aircraft. Night Fighters did it all the time in WW2. And they had to get close enough and hang on and to check "friend or foe"! 
2. Not sure where the FIR boundary was, but if it was close to their westward track (last radio call was a handover) more doubt on the ground.
3. They could have listened in to the radio calls of SIA68 with their radio. That would have given them height and position info too. So long as they did not transmit, nobody could have picked up anything from that.
4. If they had also switched off all the navigation lights nobody on the ground, or in SIA would be likely to have seen anything - rear view from a commercial aircraft is nil! Two aircraft at 35000 feet sound the same as one.
5. Timing on one other score was perfect. Night flight when radar operators are dozy, landing just before dawn - time to camouflage the plane from satellites, and a diversion of a China sea crash to confuse everyone.Good thinking, keep it up. Looking forward to more insights!