This has always been an interesting question to me, because I actively read while playing, starting many years ago when I started making a conscious effort to play from the score, even with polished pieces. I became very good a "read-playing," a skill I utilize to this day. Although I draw a sharp distinction (for myself, in my own practice) between sightreading (prima vista) and read-playing, it's clear to me that my approach to read-playing really helped my sightreading, and my strength in sightreading makes it easy for me read-play....
All that to say, when I'm playing and reading the score, I am actively looking at the notes, even with a polished piece. But at the same time, I know I'm not truly doing a "close reading" the way you would with a piece you've never played or only played a few times.
So obviously, there's a good bit that's memorized. But it's also not "memorized," because I can't play without the score.
I think for people who don't memorize, this kind of reading must be different from the way memorizers read, and also the memory we have of pieces we know well, but play from the score, is also somehow different from a memorizer's "memorized piece."
if that makes sense?
I'm curious what other habitual readers, like @twocats , think about this.