Right so similar thing happened to me about two years ago in a field near Painswick. Just this silent hovering shape, no sound at all, and that's the bit that gets you isn't it - planes and drones always have some kind of noise but this was completely silent. Watched it for maybe ten minutes then it just sort of drifted off behind the treeline.
Never fully explained it and honestly I've stopped trying. But I know what you mean about not being able to stop thinking about it, it sticks with you in a way a normal sighting wouldn't.
What sort of shape was it? And did you get any weird feeling while you were watching it, like a pressure or anything? Loads of people report that and I'm curious if it matches up with what I felt at Painswick.