I think I can smell the food inside my microwave. Is that bad?
I figured it's not necessarily unsafe, since you only need a cage to block the microwaves from escaping, but I'm not familiar with the precise architecture of a microwave oven.
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Microwave doors don't have an airtight seal; the window between the electronics and the cooking compartment is also not airtight, and the electronics are cooled by a fan. It would be surprising if some cooking smells didn't escape. In practice, every microwave I've ever had allows me to smell the cooking, from the very cheapest to some rather fancy ones with grill and convection.
This means that being able to smell the food cooking says nothing about the safety of the microwave, and you have no need to worry (unless there are other indications such as damage)
One element missing from other answers is how microwave ovens keep the waves inside. They use a Faraday cage https://en.m.wikipedia.org/wiki/Faraday_cage . The size of the holes you can have in those cages is related to the wave's frequency.
In the case of microwaves, millimeters-sized holes are ok. That allows the presence of a vent to let pressure, water vapor and smells out. In any case, if the inside was airtight, the door would blow open as the contents get hotter.
A microwave does have a sealed door, and there are no fans blowing air out of the microwave. The cooling fan(s) are not connected in any way to the chamber where you cook food. Usually there are simply some venting holes above the door, usually not visible except from above, so you may not be able to see them if it is mounted high up. As other comments have said, this is necessary to keep pressure from building in the microwave.
These vents let any food smells out rather well.