Sorry to hear of your concerns for this stray. Often cats will move away if there is a shift in patterns. So, this kitty may be a few streets over, being fed by a different person. Or it may not have been stray at all, it might belong to a neighbor that finally decided to get serious about being a responsible cat owner and has made this kitty an indoor-cat.
There is also the possibility that it got eaten by a coyote or Great Horned Owl. Coyotes take a lot of cats in the fall and during cold snaps in the winter. Great Horned Owls are starting families in Feb and so take larger prey as they prepare to nest. This would have happened even if you had been home during that time.
I think you did the best you could. Cats are independent thinkers and just don’t always fall in line with our expectations.
PK
you need to get her in to a vet.