Do you think the cat ever wonders where his poop goes? I mean, as animals, the cats do correlate phenomena and understand cause and effect to a reasonably high degree, even speculate on possible, as yet unproven causes. Phenomenon: basement door opens. Related phenomenon: I eat. Attempt to cause: If I meowl piteously and rub my face all over the canvas grocery bags hanging on the basement door and I will eat?
So the cat certainly observes the continued buildup of his own feces, is aware that he cannot always dig around the box as much as he would like to. We know he is aware as this was the cause of his second pee-outside-the-box incident.
Of course, as I type I realize the cat surely understands perfectly. He watches everything I do in case whatever I do will produce food. Phenomenon: that one fusses around in the vicinity of the box. Related phenomenon: My poop goes away. I’m sure he doesn’t wonder about it at all. Not like he wonders about the light that the oscillating steel fan casts on the hardwood floor in the late afternoon. Shimmery moving prey that his paws can never latch onto.