Why there are no three chests in Mechanar like in this video: https://www.youtube.com/watch?v=-upbUldmt38
I checked Mechanar and the Auchenai Crypts, there are no chests in both dungeons. It is unlikely that they are in all others, as it should be.
Source: http://wowwiki.wikia.com
The Mechanar, among other instances, was once famous for allowing Rogues (and some druids) the opportunity to solo their way to several chests inside the instance, boosting the clever Rogues' economy quite a bit as they didn't need to share the chests with anyone. As a result, chests within instances were removed in patch 2.2.0.