A COMMUTER ACFT FLT CREW MAY HAVE INADVERTENTLY CROSSED THE RWY HOLD SHORT LINE.

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AFTER LNDG AT STL; WE WERE GIVEN A CLRNC TO TAXI DOWN RWY 13; TURN L ON TXWY B AND HOLD SHORT OF RWY 12L ON TXWY B. WE COMPLIED. WHILE HOLDING SHORT; GND CTL ASKED US WHETHER OR NOT WE WERE HOLDING SHORT OF RWY 12L ON TXWY B. WE RESPONDED THAT WE WERE HOLDING SHORT OF RWY 12L AT TXWY B. WE WERE THEN CLRED ACROSS RWY 12L; AND TO HOLD SHORT OF RWY 12R ON TXWY B. WE COMPLIED. WE WERE THEN CLRED TO CROSS RWY 12R AND TO TAXI TO THE RAMP. DURING OUR TAXI; WE WERE ASKED TO CALL THE TWR. I CALLED THE TWR AND TALKED TO THE TWR SUPVR. HE ASKED AGAIN IF WE WERE HOLDING SHORT OF RWY 12L ON TXWY B. I TOLD HIM THAT WE WERE HOLDING SHORT THERE. HE SAID THAT THEY HAD LOOKED OUT AND THOUGHT THAT WE MAY HAVE BEEN ACROSS THE HOLD SHORT LINE. HE THEN SAID THAT MAYBE THEY HAD AN EQUIP MALFUNCTION OF SOME SORT; AND THAT HE WOULD CHK INTO IT. THAT ENDED OUR CONVERSATION. I FELT THAT WE WERE; IN FACT; HOLDING SHORT CORRECTLY. WE WERE AWARE THAT RWY 12L WAS BEING USED AS AN ACTIVE RWY. IF; IN FACT; WE WERE ACROSS THE HOLD SHORT LINE; IT WAS AN ERROR ON MY PART AND CAN ONLY BE DUE TO A PERCEPTION ANGLE FROM THE COCKPIT OR POSSIBLY THE UNFAMILIAR POS THAT WE WERE ON AT THE ARPT. I HAVE REVIEWED THAT RPT DIAGRAM AGAIN; AND FOUND THAT POS AS A HOT SPOT AT THE STL ARPT.