I definitely know what you're talking about. I don't have specific experiences I can relate it back to but I remember my old T saying on a few occasions, "I sensed that was going on" and I would sit back and think, "whoa she's good!" But I think that phrase she used really says it all. Ts are taught to listen, really, really listen. Not just with their ears, but all of their senses. Their whole job is to be in tune with you. The only thing they can't do is feel your nervous system churning out those feelings - but they can definitely gain a sense of empathy from paying attention to the other cues.

To answer your question about realizing what was happening - I don't think it's a matter of realizing what's happening, I think it's more that we reach a growth point in therapy where we start to gain a lot more awareness about what's happening in us and around us. We are doing what the T does - listening, paying more attention.