I'm by no means an expert at the piano, but I'm probably halfway there, and I can without too much trouble get the general gist of complex unfamiliar piano music, and I can easily read simple music. I'd say it's pretty much analogous to the ability of expert chess players to play blindfolded, which is definitely a well-attested ability. (The record for simultaneous blindfold matches is around 50, played by Janos Flesch in Budapest in 1960.)