Well what an afternoon that was! As I thought, as soon as I left the house Purdy slunk into view. It turns out she was hiding IN the sofa.
But the fun didn't stop there. It took the two of us two attempts to get her in the carrier. When we got to the vets she was as good as gold!
The vet couldn't find anything major. Her heart and lungs check out OK. Her teeth look fine and although she was complaining when I tried to touch her neck and jaw this morning - there was not a squeak out of her when the vet checked her out.
We were very surprised and also ashamed when he said she had a bad infestation of ear mites.

She hasn't shown any symptoms (scratching, shaking her head). They kept her in and are going to sedate her, flush out her ears, xray her head and neck and have a closer look at her teeth and throat. It looks like the mites are her problem - embarrassing and somewhat shaming for us - but very treatable. Phew.
Please folks, learn by our mistake and check your kitties ears. They might not show any signs of problems - but you just don't know what is lurking in there.
Thank you to everyone who sent good vibes our way. I'm sure they helped.