MLG ON APCH HAS TCASII ALERT. ATC SAYS NO ACFT. FALSE TCASII?

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POSSIBLE FALSE TCASII RA. WE WERE HDG 090 ON VECTORS FOR APCH TO RWY 19 AT SNA; LEVEL AT 5000 FT. HAD NUMEROUS TCASII TFC; ONE ABOUT 5 MI AT 12 O'CLOCK 3000 FT BELOW US. AS THIS TARGET DREW NEARER ANOTHER TARGET APPEARED NEARLY ON TOP OF IT; BUT SHOWING 500-600 FT BELOW US. WITH BOTH TARGETS SHOWING TCASII WENT TO RA FOR 500 FT ONE. FO ADVISED WE WERE CLBING AS WE SCANNED OUTSIDE. AURAL WARNING WAS TO CLB; SO I DISCONNECTED AUTOPLT AND AUTOTHROTTLES; ADDED PWR AND STARTED CLB. SCANNED FOR GREEN LIGHTS ON VVI AND THERE WERE NO LIGHTS. REALIZED THERE WAS ALSO NO LONGER A CLB COMMAND; SO LOOKED AT TCASII AND THERE WAS NO 500 FT LOW TARGET. STILL HAD 3000 FT ALMOST DIRECTLY BELOW US. STARTED RETURN TO 5000 FT AND ADVISED SOCAL OF SAME. CTR STATED 3000 FT BELOW HAD SET OFF RA; BUT WE DISTINCTLY HAD 2 TARGETS. NEITHER FO NOR I EVER SAW IT VISUALLY HOWEVER. AFTER LEVEL AT 5000 FT; TARGET AGAIN APPEARED; NOW BEHIND US AND 600 FT ABOVE. DID NOT TRIGGER TA OR RA. PREVIOUS TO ENCOUNTER I DID SEE A LARGE; GRAY CARGO VESSEL WHICH WOULD HAVE BEEN ALMOST UNDERNEATH US DURING THIS TIME. ATC SHOULD KNOW THAT TFC 3000 FT BELOW WOULD NOT TRIGGER TCASII.