Sometimes it isn't the cat so much as the house that is to blame; when I lived in an old apartment building, my cats removed the poorly attached plumbing panel in a closet facing the bathroom, and then hid under the tub. If the momma-cat got out of my apartment front door, then she was as good as outside, even though the common stairway entry area was behind a good, stout door. The water heaters were in a closet, and the momma-cat found the way from the entry hall, into the closet (with poorly hung doors that didn't close), and outside.
I'm so happy for you that Yon is back! I'd try to do some detective work to find out where she went, though, because it might be dangerous. I finally did repair work around the apartment (landlord was terrible at that kind of thing) after one of the kittens died. I don't know where she got to or what she got, but she was very ill one day, and my vet thought it was probably some household poision. It was more likely poision behind the oven than poision under the tub, but neither are safe places, and it's very hard to know how safe or dangerous Yon's hiding place was.