house detective

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A detective employed by a retail store, hotel, or other establishment to prevent theft or misconduct by patrons.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person employed by a privately owned establishment, such as a hotel or large retail store, with the job of preventing wrongdoing and apprehending violators of laws or other regulations.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a private detective employed by a hotel or retail store