The remains were found Tuesday in Bolingbrook by a surveyor who discovered the body near Interstate 55.
Bolingbrook Police Lt. Mike Rompa told the Joliet Herald-News that the man checked himself out of Adventist Bolingbrook Hospital "against the medical staff's advice" on Nov. 10. He left on foot and was later reported missing by a friend.
The Daily Herald reports an autopsy will be performed later this week.
Bolingbrook is about 25 miles southwest of Chicago.