Oneslip

Hello, I have an 01 C320 & lately when I have started the car the defrost blower kicks on and air is coming from the center vent. The AC is not on at all, the screen is at -0-. It will run for a minute then kick off. Today I started the car to pull it out so I could get my motorcycle out & when I pulled the car in the garage and shut off the engine, took the key out the blower kicked on & ran for a minute. The AC is still off at -0-when this happens. Can anyone explain what is going on with this or is this a normal function. Is there a sensor that can detect humidity? Would appreciate any insight into this before I call the service rep at MB. Thanks everybody, have a great weekend.