MLG HAS A TCASII RA WITH AN UNSEEN ACFT IN THE ARSA WITHOUT CLRNC.

Narrative

AFTER BEING VECTORED TO A L DOWNWIND FOR RWY 5 AT ORF; WE WERE CLRED FOR THE VISUAL APCH. WHILE TURNING BASE LEG; OVER THE RIVER (AS PUBLISHED IN THE NOISE ABATEMENT PROC); WE OBSERVED A TA ON OUR TCASII. THE TFC WAS AT OUR 10 O'CLOCK POS; LESS THAN 3 MI AND LEVEL AT 2000 FT. AS A RESULT; WE LEVELED OFF AT 2500 FT TO LOOK FOR THE CONFLICT BUT WE NEVER MADE VISUAL CONTACT. IMMEDIATELY THEREAFTER; WE RECEIVED AN RA WHICH REQUIRED A 3000 FPM RATE OF CLB TO CLR THE CONFLICT. WE INFORMED THE TWR OF OUR REQUIRED ACTION. APPROPRIATELY; THEY TOLD US TO CONTINUE A GAR AND VECTORED US TO A R DOWNWIND FOR RWY 5; ANOTHER VISUAL APCH; AND UNEVENTFUL LNDG. IN CLOSING; ATC HAD NOT WARNED US OF ANY POSSIBLE TFC CONFLICTS. EVIDENTLY; THE TFC DID NOT APPEAR ON THEIR RADAR SCREEN UNTIL AFTER OUR RA ADVISORY. FURTHER; ATC SUGGESTED THE TFC HAD ENTERED THE ARSA WITHOUT NOTIFICATION AND/OR CLRNC. LASTLY; WE ARE GRATEFUL TO TCASII AND FEEL IT DEFINITELY HELPED TO PREVENT WHAT COULD HAVE BEEN A TRAGIC EVENT IN ORF.