User blog comment:Decca03/Scooby-Doo! Stage Fright – Review/@comment-252417-20130614060038

There was a trap involved and there was an unmasking and I think that a Scooby Snack was switched with the diamond (somehow a Scooby Snack is as heavy as a diamond).

If you recall in Scooby-Doo And The Alien Invaders there was not a trap either (although the aliens did catch some of Mystery Inc. in a net) and the villains unmasked themselves (right after that trap scene).