If your heart is saying take her in...do it.
You might try putting a collar on her and seeing when she comes back if it's still there. I know I'd be a little shocked if my cat turned up with a collar on all the sudden. You could even put a tag with your number on it. At least then you'd know?
Feral cats are tameable. I've seen it. I work at a vet clinic and it took us two years to convince Mama kitty we were ok. They trapped her with a haveaheart trap and spayed her. No more unwanted babies. Now she sleeps in one of the cages at night and plays outside during the day. She even greets me every morning by jumping on the hood of my car and begging for a scritch. God forbid I don't or she'll jump down and smack my ankles. She also loves to play with my dog.
I have four indoor kitties already or she would have been another addition. I have said however that if/when Raven (my eldest) time comes I will seriously consider taking in Mama Kitty.
It may take some time but many ferals can be tamed with patience.
However with the quickness this one was tamed...I'd say she either has a home, was dropped off, or wandered very far and is now homeless.
Good luck
