When Grandad is trying to play draughts on the talking chess game, the voice on the game talks before he presses the button.
The briefcases had the note of the combination on the inside rendering them un-openable. They only have a 3 digit combination so it would have only taken a maximum of 999 seconds or 16 minutes 39 seconds to open each lock trying one combination a second. Therefore, they wouldn't be un-usable anymore.