Set up the lockers in the stadium the night before the game and, even if they weren't, the equipment should have been at the stadium that night (even if still packed). So, someone who had keys for the stadium could have gone in and vandalized the equipment overnight.

I find this version very questionable, though. First, you would expect, on finding it before the game, that BYU would have removed the red dot before the game. Second, the prank would have made the news -- news outlet would love reporting on this type of prank. Third, it would have been a scandal for the Hawaii stadium, and likely the University -- people would have been fired over it and, even though they likely would have wanted it kept quiet, this isn't the type of thing that typically can be kept quiet.