AJ Khan

Hi there I have a very confusing situation my C 2oo 1998 auto sedan turn it’s exhaust fan on by itself full speed while the keys are out in middle of the night when temperature drops 13c / 14c doing it since I washed it and washed the motor and engine bay with degreaser but dried it with pressure air hose have done it to all my car now and then for past 45 years so please, what is it ,I cleaned all the connection leading to fan now I am stopping by removing the (fuse 38) RACV was called they didn't know took to my mechanic he failed to diagnosed it. Please help.