Hideout is a location in Paradise Lost. It's a hideout for Bandits lead by their leader Two-Ears. Formerly it was the Parcel Center and therefore player can see several posters connected with mail service.
The reason why Postal Dude must go there on Thursday is because he was robbed by two Bandits who took all his items along with Ensmallen Cure. After Postal Dude enters into their Hideout, he notices that his stuff lying not far from main entrance, and rest of them can be found in the whole building. However, to reclaim Ensmallen Cure, Dude must fight with Bandits' leader Two-Ears along with other Bandits.