Confusion; Distraction; Communication Breakdown
Conflict NMAC
General None Reported / Taken
Procedure
[We were] cleared direct to MOD (or OAL) for SFO arrival. We were either in descent or temporarily level preparing for descent. At some point TCAS displayed 'TRAFFIC; TRAFFIC' and yellow TCAS showing target 300 FT above our altitude and descending; from right to left; nearly directly above us and moving VERY quickly. TCAS showed approximately 2 dot (target) movements (thereby giving indication of FAST movement) before disappearing from scope and never reappearing. Crew immediately looked for traffic visually but saw nothing. No RA. Crew assumed it was a spurious TCAS error and continued arrival. After landing; a passenger asked the Captain if we experienced a near midair collision since he visually saw an aircraft out his window VERY close and rapidly heading away to left at about the same time we had our incident.
The Reporter stated that after the fact the crew assumed a military aircraft was involved because they were near the Sierra Nevada Range and at times there are military aircraft in that operate VFR in that area. He emphasized this was a guess on his part.
A First Officer reported a TCAS TRAFFIC alert and near miss at FL200 while direct to MOD for a SFO arrival but the crew assumed a TCAS error until a passenger later reported seeing traffic pass nearby.