SIMON COWELL Robbed During One Night Stand

Simon Cowell has learned the hard way not to sleep with strangers, because when he woke up he realized he had been robbed of a computer that contained X Factor secrets.

TMZ has confirmed ... last October, Simon hooked up with a girl he met at Drai's nightclub in Hollywood. He took her home, but the next morning she was gone and so was his computer and his wallet loaded with cash.

Simon -- who has his Bev Hills mansion wired for security -- called the cops.  The footage showed the woman, cops tracked her down and she handed over the computer and the wallet sans cash.

Although it's unclear exactly when Simon broke up with fiancee Mezhgan Hussainy, sources connected with Simon tell us it was BEFORE the theft.

Fiancees and strange girls can be expensive.