the microwave was on top of the fridge, took it down and cleaned the top off properly for the first time in 10 years. Was covered in dust. Underneath the dust was some yellow stains, like a mouse had been peeing there. Weird. I have tried to wipe it down occasionally but I think this is the first time I've ever cleaned it properly. The yellowish might also be residue from the air freshener I had on it years ago.