ACR PIC RELEASES PARKING BRAKE AT GATE ON ARR WITHOUT CHOCKS. ACFT ROLLS BACK UNTIL GND CREW ALERTS CAPT.

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AS WE TAXIED INTO GATE IN LGA THE GND AGENT MARSHALED US IN WITH WANDS. AS WE APCHED OUR PARKING STOP; HE BROUGHT HIS WANDS TOGETHER AND THEN HELD THEM CROSSED; OVER HIS HEAD; INDICATING WE SHOULD STOP. I SET PARKING BRAKE AND SHUT DOWN ENGS. THE RAMP AGENT WALKED TOWARD THE JETWAY; TOOK A WAND IN EACH HAND; HELD THEM OVER HIS HEAD AND BROUGHT THE TIPS TOGETHER. (THIS IS THE GENERALLY ACCEPTED 'CHOCKS ARE IN SIGNAL.') I THEN RELEASED THE BRAKE AND CALLED FOR THE PARKING CHKLIST. UNKNOWN TO ME; THE ACFT HAD STARTED ROLLING BACK; GND PERSONNEL BANGED ON THE SIDE OF THE ACFT; AND I BROUGHT THE ACFT TO A STOP. THE RAMP AGENT DENIED HAVING GIVEN ME THE 'CHOCKS IN' SIGNAL. AFTER CONSIDERATION I CAME TO THE CONCLUSION THAT HE EITHER WAS SIGNALING THE OTHER GND CREW TO CHOCK THE ACFT; OR WAS SO USED TO GIVING THIS SIGNAL AFTER STOPPING THE ACFT; THAT HE DID IT WITHOUT THINKING. IN ANY EVENT; I WENT TO OUR OPS MANUAL AND FOUND TO MY DISMAY THAT THERE IS NO OFFICIAL 'CHOCKS ARE IN' SIGNAL. WE HAVE ALL BEEN OPERATING UNDER A NORM WHICH IS NOT PUBLISHED AND IS NONSTANDARD. WE NEED TO EITHER INCORPORATE THIS SIGNAL IN OUR MANUALS OR PUT OUT A LETTER TELLING OUR PLTS NOT TO RELEASE BRAKES UNTIL WE RECEIVE THE 'RELEASE BRAKES' SIGNAL.