I've seen this with my ISTP. It's nice and mellow and chill and hard to believe because it really does look like you're in deep thought. If you're used to hyperactive or high strung people, it can take some getting used to.
I've been there before, just staring off in to the distance, feeling the wind and sun on my skin...nothing going on in my head-just experiencing. People usually walk up to me like mentioned, and ask what I'm thinking about. When I say "nothing" it goes one of two ways, the other person just says, "Alright," and that's that or they just nag me for 5 minutes saying, "No really, you can tell me, what were you thinking about?" *facepalm* <------Probably one of the most annoying things someone can do-not take what I say at face value.
As far as the OP goes, any kind of noise/sight/touch distracts me. Like you said, flashing notifications. Things like, blinds waving back and forth, light flickering, birds chirping etc.