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.

Here it is ... the $8 million "parting gift" Simon Cowell gave to his ex-fiancee Mezhgan Hussainy when he dumped her ass earlier this year -- his old, used mansion in Beverly Hills.

TMZ has learned ... Simon moved out of the home a few years ago ... when he upgraded to a spiffier 6-bedroom $12 million dream home ... just a few miles away (below).


As we previously reported, Simon felt Mezhgan "deserved" the old property for having to deal with all of the public scrutiny that comes with dating the "X Factor" honcho.

The real question -- why did Simon have an old mansion just lying around? If you have to ask, you can't afford it.

Simon Cowell is telling friends ... his engagement to Mezhgan Hussainy was a "big mistake" ... but she left the relationship with one fabulous parting gift ... an $8 MILLION MANSION.

The details surrounding the breakup are unclear ... but according to TheSun.com, Cowell has admitted, "I'm a hopeless boyfriend."

The two got engaged back in 2010 after meeting on the set of "American Idol."

Sources close to Cowell tell us he feels Mezhgan "deserves" the home for putting up with all of the publicity and scrutiny that comes with dating the "X Factor" honcho.

We're told the two are still on good terms ... probably because it's hard to hold a grudge when you're living in an 8 MILLION DOLLAR MANSION!

Cowell has a history of showering his exes in riches following a breakup ... his former flame Terri Seymour reportedly walked away with a $5 million mansion and millions in cash when they split back in 2008.