> I'm guessing this is a sound made by the piano, but I'd love to know for sure.<<
I know the recording well, from waaaaay back, and grew up watching and recording great pianists rehearse at my childhood home--my parents were music presenters. Based on that, here's my hypothesis.
That sound, which only exceptional gear can "separate" from the music and other noises, is Nojima's fingernails clicking on the keys! Is suspect KOJ's outrigger omnis, which he used back then for a little extra sweetness, were a tad close and picked up the clicking.
The fact that they seem to float a touch wide of the rest of the music is a function (I'm guessing here) of that closeness and the frequency. The omnis did their job picking up "room," and turned it into directionless space. However, they also caught the clicks which are, even with low-volume spaced omnis, localizable.
