The microwave fan will sometimes come on when your not using the microwave. It does this when your cooking something on the cooktop below the microwave. Boiling potatoes or pasta or something that is creating steam is usually the culprit.
The thermostat senses the heat and turns the fan on.
You should have the microwave checked out by a qualified technician. A survey test should be performed by technician every time it is serviced.
Remember, the capacitor inside a microwave can hold 4,000-6,000 volts even after its been unplug for days and even weeks.
This means that a microwave doesn’t need to be plugged in to KILL you.