Microwaves have doors that seal like an airlock on a spacecraft. Remember that when you leave food in there thinking you will heat it tomorrow - a friend of mine did that and forgot about the takeaway chinese meal she had left in her microwave...
Two weeks later she opened the door and nearly passed out from the smell.
It is important to remember that out of sight is very much out of mind with microwaves. Just because the door is shut and you can’t smell anything, doesn’t mean that everything is ok.
I clean out my microwave with a cloth dipped in water and rung out. You can use a solution of disinfectant such as Dettol in water, or a solution of vinegar and water.
It is best practice to wipe around the microwave each time you use it as smells tend to linger, particularly from fish.
If you have a problem with a smell in your microwave, put half a lemon on a microwave dish or plate and cook on high power for about 15 to 30 seconds - this will release a lemon smell into your microwave that should take care of any lingering food smells.
Jeep the door open after cooking or cleaning to allow moisture to disperse.