No, I've never noticed a therapist mimicking my body gestures (thankfully). It would definitely creep me out.
However, on several occasions I caught myself mirroring the body gestures of one particular T and this freaked me out just as much. I wasn't doing it deliberately. It was subconscious. On 2 or 3 occasions, I just suddenly became aware of it and became super self-conscious and embarrassed. I don't know whether T noticed it. I'm guessing so but it was never brought up. I don't even know why I was doing it because I don't think I have ever behaved in that way before with another person.
I understand that T's sometimes mimic deliberately to tune into how their client feels or to try and communicate empathy but what if it's not deliberate or it's the opposite way round and it's the client that's mirroring?