It is interesting, isn't it. 
I have a couple of questions that I want to ask DGD about this next time I speak to her.
The first relates to colds. I remember that my children, when young, used to have a very specific odour ( that seemed strong to me) whenever they had colds. I am wondering if she has ever noticed that smell on the family, and if the covid smell is similar.
The other is how long she could smell it for. Her dad was still testing positive for covid over two weeks after it began - could she smell it all of that time?
Oh, and I've thought of a third question. Did she smell it on her brother, who only tested positive for a few days?
*Chestnut*
" Just out of interest why not try testing her sense of smell to see if she can pick up subtle smells. "
That's an interesting idea. She does seem to have quite a sensitive sense of taste (picky eater), so it wouldn't surprise me if her sense of smell is similar.