After checking the dustbin and the refrigerator, we opened the microwave and found where the smell had been hiding.
It had been infusing itself onto the sides and the glass-turn-plate-thing, it had been settling into that space as though it wanted to build a home for itself there. The microwave was harbouring a fugitive odour.
The smell had to be defeated. Initially we ignored it, hoping it would just disintegrate. However, it was persistent. It lodged itself in there.
We tried bicarbonate of soda, we tried the small smell-removing penguin that lives in the fridge. No no, both of them failed.
So trusty lemon came to the rescue: sliced, microwaved on high in a water-bath for 5 minutes, it removed the fishiness and replaced it with a pleasant lemony-fresh smell.
So there you go: if something smells strange, rub it down with a lemon.