NMAC. PLT OF A BEECH BONANZA TOOK EVASIVE ACTION BY CLBING AND TURNING TO AVOID A MOONEY OBSERVED FLYING UNDERNEATH HIS ACFT 200 FT BELOW. SUBSEQUENTLY; RPTR PASSED SMALL LATEX BALLOONS ON FINAL WHICH WERE RELEASED FROM A DOWNTOWN FESTIVAL.

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MAKING APCH TO SJC VIA RHV ARPT. IN CONTACT WITH BAY APCH. 3 CONTACTS CALLED BUT MOST LIKELY ACFT WAS A MOONEY (VISUALLY IDENTED) WITH CALL SIGN ENDING IN YYY OR XXX. TFC CALLED AT 10 O'CLOCK POS (MOONEY) 2300 FT. OTHER TFC CALLED AT 12 O'CLOCK POS; ALT UNKNOWN. TOTAL OF 3 TARGETS CALLED. ACTIVE SEARCH FOR TFC DID NOT IDENT TARGETS. A MOONEY FLEW BENEATH US FROM 2 O'CLOCK TO 8 O'CLOCK DIRECTION WHILE WE WERE FLYING A HDG OF 300 DEGS. OUR ALT WAS 1500 FT. TERRAIN IN THE AREA (RISING HILLS) WAS AT LEAST 800 FT. ACFT SEEN APPROX 1-2 SECONDS PRIOR TO PASSING BENEATH US. EVASIVE ACTION TAKEN BUT PROBABLY NOT NECESSARY; IE; BRIEF TIME PERIOD TO RESPOND AND UNCERTAINTY OF CLB ALT RESULTED IN SNAP CLBING R TURN. BAY APCH SUBSEQUENTLY NOTIFIED THE MOONEY OF US (BONANZA) THAT TFC WAS NO LONGER A CONSIDERATION. (MOONEY WAS BOUND FOR SOUTH COUNTY ARPT.) EXCEPT FOR THE FRIGHT FACTOR; NO INJURIES OR DAMAGE TO ACFT WAS IDENTED. THIS IS A HIGH TFC AREA. HUMAN FACTORS: 2 PLTS LOOKING FOR TARGETS WITHOUT CONTACT OR IDENT. WE WERE GENERALLY LOOKING ABOVE US AND LEVEL SINCE CALLOUTS WERE FOR HIGHER ALT AND TERRAIN DID NOT SUGGEST POSSIBILITY OF AN ACFT BELOW US. BAY APCH WAS BUSY AND THE OTHER ACFT MAY HAVE BEEN BELOW RADAR OR WAS CLBING FROM RHV AND HAD NOT BEEN PICKED UP. THE OTHER POSSIBILITIES INCLUDE THAT IT WAS THE ACFT THAT BAY APCH IDENTED BUT THEIR MODE C WAS NOT ACCURATE. LASTLY; ON FINAL APCH TO SJC; MULTIPLE SMALL LATEX BALLOONS AROSE FROM A FESTIVAL IN THE DOWNTOWN AREA. NO EVASIVE ACTION TAKEN ALTHOUGH ONE POPPED UP AND PASSED BY US APPROX 15-20 FT ABOVE THE PROP. IT APPEARED TO BE A CONCERTED RELEASE ALTHOUGH I CANNOT EXCLUDE HAPPENSTANCE RELEASE BY COINCIDENCE. I SUGGEST THAT RELEASES NOT BE ALLOWED ALONG THE APCH PATH.