As I was drying Petunia's hair tonight, she stopped to consider for a moment and then announced out of the blue that the hum of the hair dryer was a D. As in the musical note. (According to the piano teacher, all three of my children have perfect pitch.) Amused and curious, I walked down the hall into Thing Two's room, where he was lying quietly and obliviously on his bed reading, and sang the same note to him. "D," came the instant reply.
I once asked Thing One if he's consciously aware of all the musical tones played around him on a daily basis (the 'ding' of elevators, the jangles of doorbells or wind chimes, the humming of mechanical equipment, etc) and he said no, something has to draw his attention to them, although once he does stop to pay attention he always just somehow knows what the notes are. It's like he has an internal reference standard in his head.
Incidentally, not one of the three of them really 'gets' that the vast majority of people can't do this. When they answer me, there's always a baffled element of "Why can't you tell? Isn't it obvious??" lurking in the background. The brain is a truly fascinating place.