I had - sadly - given up on Jemima. Friends down the road had a cat simply vanish a while ago. He didn't turn up one day, and they never found out what had happened.
Ten minutes ago I heard a mew from the kitchen and thought it was Sophia (Jemima's twin) asking for something. Imagine my surprise and delight when I saw Jemima herself at the food bowl! I picked her up to cuddle her, and she felt very light, but then she's always been the lightest of the four cats.
However she's not happy. One of her eyes looks as if it's been in an injury: perhaps she was in a fight, or caught it on a tree branch or something. It's very weepy, but she won't let me go near it. She's sitting on the floor under my desk at the moment, and doesn't want to be stroked. What's more, I gave her some yogurt and she didn't want it, which is most unlike her.
Cats usually recover from injury or illness on their own; it could be, as Tim suggested, that she wasn't well and has been taking some time out comvalescing away from the family. In which case, she must be better to have returned. But of course if she's still miserable tomorrow, or if the eye looks infected, we'll have a visit to the vet.
I'm still smiling at her having reappeared, though.