Takehertakehertakehertakehertakehertakehertakeher ...

You are justified in being angry.

Squirt needs you.

Squirt needs veterinary care.

They need neither an explanation nor to be paid for Squirt. Keep wandering by until you see her outside some day, cat-nap her, then send them a note saying you were in the nieghborhood, saw her outside (again) and were worried about her fate. End of story.

I doubt they will ask for her back.

And you will then have a sweet little kitty whom I bet will just love you to pieces once she is well again and realizes what a good situation she's gotten herself into.