Big Sean had his Hollywood Hills home burglarized last week, while he was out of the country.

The thieves made off with $150,000 in jewelry and -- more importantly -- some of his unreleased music.

Sean thinks it was an inside job, and we just learned another detail of the crime: The robbers left a note telling Sean where to contact them if he wanted his stuff back.

Bad idea.  Sean has turned that information over the LAPD, who now have a special unit trying to track down the thieves.

Wouldn't it be something if they turn out to be Ariana Grande's goons?