Will, I don't doubt you can pick out the right key on a guitar audibly, like I could find that Bagpipe A, but with guitar I wonder how much of it is hearing the chord voicings and the sound of open strings vs stopped strings? As you know E major in open position sounds considerably different from the same name chord played in the open D position with a capo on II.

That's a good point! AS I only use a capo now and then, it may well be the voicings I hear in my head that I'm very familiar with - I must try the experiment...

Many years ago, I went to a guitar "setup" workshop in Romford run by a chap called (if I remember correctly) Robbie Gladwell. A bunch of us sat round while he explained how to work magic on a fretboard. He tuned up a few guitars from scratch with new strings - and got them up to correct pitch without any tuner whatsoever. I asked him if he had perfect pitch - he said, no, he was just used to it as one of his jobs was setting up all the Gibsons imported into the UK.