I do notice these both in my and others' reactions. Far from only in therapy sessions but, as pointed out, some therapists consciously apply of this very general phenomenon that we use but most of the time are not aware of in everyday life.There has been a lot of fascinating research on behavioral mirroring and how it relates to certain mechanisms in the brain and its importance in communication and even evolution. There are many articles on the web, just google things like "mirror neurons, mimicking, empathy, transference".

Here is a simple one:
MIRROR NEURONS-CELLS THAT READ MIND | RoboZine

The thing with the necklace is interesting though. I think I could not resist and ask the T about it directly.