I have a young cat (9 months) who was raised indoors and has never had the chance to observe other cats using the dirt outside as a cat toilet. Until now, she has never shown any concern about her litter box - she has been OK with me filling it with whichever litter brand I bring home, and I only have one (it is a small flat, I don't know where I could place a second one) but she doesn't mind using it multiple times a day before I clean it. Except for a single accident when she was a young kitty the first week in my home, she has never urinated or defecated outside the box.
She has been going out on her own, in stages, for several months now - at first I would walk her out on a leash, then bind her on a long leash on the patio and leave her there for a couple of hours, then open the door for her to roam freely, and for about a month now, she has a cat flap and comes and goes as she pleases. In all that time, she continued using the litter box as usual.
Three days ago, I refilled the litterbox. It was a different brand than last filling, but I cannot say for sure if I have used it before. Since then, I have only scooped out one clump of urine, and no excrements at all. The cat is not urinating or defecating in the flat, I would have noticed the smell even if it was hidden somewhere.
One possible answer is that she, all of a sudden, learned to go to the toilet outdoors. I don't know what would have made her do so, and I don't see signs of digging in my few soft-earthed flowerbeds. Of course, she could be going in the neighbours' gardens :)
The other possibility, which worries me a bit, is that she is ill, or dislikes the new litter so much that she is just holding it all in. She doesn't show outward signs of being ill, eats in mostly typical amounts (her appetite is not yet completely stable, I suspect growing phases), and shows no irritability.
So, are the bad scenarios real and/or likely? Also, how likely is it that a young cat, from one day to the next, learns to go to the toilet outside and starts preferring it to the point she doesn't use the litter box? With the flap, she still sometimes stands in front of the patio door, asking me to let her in, so in some sense, she has her habits :)