I have operated SIN LHR for the last few years routing over KL, Port Blair then over Calcutta. The new info about this flight being flown over this similar route makes we wonder if it "tailgated" such a flight heading towards India.
SQ068 was speculated as being a possible "Shadow" many pages ago. But nobody ever confirmed its actual flight path that evening. This was not an amateur job, whatever happened, so I wouldn't rule that out.