We've learned Simmons DOES leave his Hollywood Hills mansion we thought was his entire universe for the last 3 years. We're told the disguises are not always elaborate ... sometimes a hat and street clothes do the trick.

As for where he goes ... sometimes lots of people see him but don't notice. We know he's been to the Beverly Center, a bustling shopping mall near his home.

We're told Richard is walking with a cane, because he's still dealing with his knee injury. He's gained some weight, but our sources say he sounds "totally normal."

Richard has been temporarily sidelined ... he checked himself into a hospital Monday for a gastrointestinal problem.