US carriers at US airports do use the boarding pass designator code to identify those to be searched a second time. I don't believe AC does on either side of the border, though. boacvc10, my condolences. Three times! What could you have hidden on your person or in your case between being checked, boarding, disembarking and reboarding? STUPID, DUMB, ASSININE... Only in America, where common sense is now considered subversive.
BTW, did you actually fly on a VC10 on BOAC?