Condensation in or around the microwave.
At this time of year as everything gets colder, there is more chance of condensation forming as you use the microwave, either inside the oven or on the worktop around the oven especially if you use the oven for long cook times cooking casseroles, potatoes or rice.
Once you have finished cooking, if necessary, wipe around the inside of the cavity and door of the oven with a kitchen towel or cloth (don’t forget the roof). If condensation is left eventually the oven will rust and become unsafe to use.