One day, the police receive an anonymous tip that a robbery is about to occur on the fortieth story of a tall building. They grab a car and drive as fast as they can to the site. On arrival, they quickly cordon off the building and rush to the fortieth floor on the elevator. However, upon arrival, they find nothing amiss. To be safe, they disguise themselves and surreptitiously stick around for an extra two hours but eventually return back to the station. However the next day, the police receive a call from a frantic man who had returned from a trip to find his flat ransacked and a taunting letter from the alleged thief. The police are left perplexed--how had the thief avoided detection?
edit for more information: The robbery occurred during the middle of the day. The information given in the tip is accurate. The frantic man who called the next day had indeed been robbed by the thief. He was the only person who's room had been robbed that day.