Matchmaking system finds people that are approximately on the same level as you are (+/- 5 levels). All players above level 51 are treated like level 51 thieves. When there are no more players available in this range during skipping (all of them are under shields, online or already skipped), the game starts to show fake dungeons based on real ones. Gems and gold in these dungeons are fake. An owner of the original dungeon will get no notification and he will not lose anything due to attacks.

This is done to prevent the situation in which you have no one to attack for a while.