I believe you said originally that she initially stayed outdoors most of the time. Cats who live outdoors have a different idea than you do about belonging to people. They simple adopt a territory and consider it theirs because food is available and the humans are non-threatening. This is a feral cat habit. You may think you are bonding, but in fact you are generally just a pleasant object in their territory -- that is if they will allow you to pet them. They may even get in the habit of calling you to the door, greeting you with rubs and purring, etc., but they have not really bonded. You are a convenience.
Likely scenario -- one day, while out hunting or being cased by dogs or other cats, she came upon another house with some friendly people and she stayed around there to take her meals because her own territory had become insecure. After several years, those people moved away, or some of them did, leaving humans who were less interested in putting out food. Maybe they acquired a dog that frightened her and made coming near for food extremely stressful. So she thought back to how it was at your house -- nothing near the house threatening - nice people -- familiar hiding places.
So one day she comes across a pathway or scent that is familiar, follows it a ways, finds another familiar place, follows that, and lo and behold, there is your house. Food, welcome, friendly humans. She is not stupid. She is getting older, her experiences have tempered her independence somewhat, and she has perhaps had several narrow escapes within recent weeks. Once she is in the house, she discovers it is not such a terrible place, and the food is good, and the folks are friendly inside there too. Now finally, combining her territory (the house) with her food source and humans who speak nicely to her, she begins to feel real security and companionship. She is intelligent and reasonably analytical. The woods and the far-places hold many terrors and traumamtic experiences. The house and its immediate yard is an oasis of security and regular food and shelter and security.
If you reinforce that with a great deal of personal, hands-on attention (carefull not to move in on her too fast), she may truly bond with one or two people in the house. Then it is unlikely that she will go off again.
Of the 30-odd cats that have passed through my care over 3 years and more, I usually retain about 14-16 cats who are bonded to me and not to territory. I have 5 other cats right now who are bound to the area and who come in the windows from time to time, say hello, grab a meal, and go off again. Occasionally I have seen some of the cats who never come back, and most of the time they chirrup at me and may even come over for a sniff and a pat, and then they go about their business. Cats, like people, often are independent of mores and norms and may simply want their own space on their own terms. Just the way it is.