A hotel detective is a plain clothes person engaged to monitor the security of a hotel and investigate various security or rule violations therein. This is not the same as uniformed security guards employed by a hotel.
Hotel detectives are often retired and/or ex-police officers with some training. They have prominence in certain noir fiction, especially in the works of Raymond Chandler, and sometimes referred to as "House Dicks". The term "hotel detective" is no longer used as most hotels today employ uniformed security staff who leave investigations to law enforcement.