** Whenever I use the vent fan on my over-the-cooktop microwave, I later find grease all inside the microwave compartment. Why is this happening? **
I don't see how it's possible. The air for the exhaust should have nothing in common with the air circulated though the microwave and it would have no way to get into the microwave without the microwave itself being on.
Maybe the vent is blocked causing the exhaust to back up into the cooling vents for the microwave and from there being forced into the oven cavity?? I really can't see that happening myself.