So, found our poor girl.
Before going to do blacksmithing with Hugo, I decided to do another check of the perimeter (she wasn't there yesterday) and she was curled up out back. She let out this quiet meow when I went up to her. VERY weak, but she still tried to stand up to greet us when Julia got there as well (called out for her, as I didn't want to move her until we checked her for injuries).
We figure she got locked in a garage, and had no food and water. It's a theory that she was making the trip home in small bursts, and when we found her she'd collapsed there, and we got to take her the final leg home. Homeward bound anyone?
Took her inside, fed her small amounts of food, sprinkling of biscuits, three small pieces of steak, and then put her in her own quiet room with water and a tiny bit of food.
Mum loaned me some money to see the vet, and vet said we were doing everything perfectly, and that Stella was coming along nicely, surprisingly well in fact.
Got some rescue food for her, and going to check her through the night (every 2 hours), and give her small amounts of food. She did have a bit of a bung walk, but that's most likely exhaustion. We'll see how she is tomorrow. For now...she's safe, and sleeping. One very tired puddy tat. Most just leaving her be to de-stress and mend.
Relief... Thank you for all the thoughts and support...