A first officer reported a TCAS traffic alert and near miss at FL200 while direct to mod for a sfo a...

Narrative:

[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.

Original NASA ASRS Text

Title: 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.

Narrative: [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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1034610
Time
Date	201209
Place
Locale Reference	ZOA.ARTCC
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Function	First Officer Pilot Not Flying
Events
Anomaly	Conflict NMAC

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.