Quest Locations are special, limited time locations that are only available to visit during certain quests. Once the quest is complete or the quest timer runs out, these locations and any items inside them will disappear.
They can be accessed by clicking on the Locations button. If a quest location is available, it will have a timer and the quest icon above it, and say "Quest". These locations first appeared with the Mystery Mansion Week theme in September 2012.
Other Sims are unable to visit these locations, and if they attempt to visit you they will be placed at the last house you visited (Beach House, Home, Penthouse, Midnight Grove, Winter Chalet, Kingdom of Simelot).