You mention unmoving shadow and a shade of light at a specific time.
This reminds me a lot of my own cat.
He has always had a fascination for these things and a keen memory for it.
He always comes sit with us at the dinner table, because the light reflects on our knifes against the ceiling, he loves this and whenever we eat, he will sit and stare at the ceiling, even if there is no reflection "active".
He also spots any other reflections nearly instantly.
He also loves moving shadows. Is the fruit basket always there? it could be that he is intrigued by the "new" shadow on the wall.
My other cat who is 1 year older never shows any interest in any of these things, he will not chase a reflecting light on the wall or ceiling and cares even less for shadows.
So interest in things depends on the cat.
I wouldn't worry to much about it but i understand you as an owner, you have to remember that these animals do not think like we do and that everything new does not have a simple explanation for them.
We know at night that light from outsides comes from carescars, streetlights or even people. Cats do not.