I've had lingering odors in a microwave before. Burnt popcorn is not a smell I enjoy and even if the fish cooked perfectly, the smell lingers a bit longer than I want it to. I thoroughly clean it out (top sides bottom and the seals around the doors) (if there's a removable glass turn table, wash it in the dishwasher) first with spray on grease cutting cleaner and when it's all clean, go over it again with baking soda and vinegar paste, then wipe it clean again. Follow that all up with boiling a cup/bowl of water with lemon juice in it then let it set in there till it's cool again.
In the end the microwave comes out scent free every time.
When cleaning out a fridge (emptying it and giving it a complete wipe down), the baking soda vinegar paste works wonders in there, too.