Buffalo Grove police and fire units responded to Woodman's Food Market at 1550 Deerfield Parkway just before 3 p.m. for a well-being check on a person inside a vehicle emitting a foul odor.
First responders found a man dead in a blue Chevrolet Equinox parked on the south end of the parking lot. His identity has not been released.
A large fire and police presence was on the scene for more than three hours.
The Lake County coroner was called to the scene to examine the body.
Authorities are conducting a death investigation, and police believe there is no threat to the public.
Police did not immediately provide further information.