This happens to me too. Sometimes it's completely unrelated and I'm like wtf!

My T is pretty good about noticing when it happens and she'll sometimes ask me what I just thought about or 'where did I go'. Usually she'll sit back and let me go with it and then when I come back, she asks about it.

Sometimes I don't want to talk about it or I'm not ready to. Other times, I try to make the flashback stop. I can sometimes pull out of it if I dig my nail into my hand. For some reason that helps re-ground me I guess.

My T does see this, but she lets me decide whether or not to bring it up usually. There are times when I do want to tell her, especially if it scared me, but I usually can only do that through writing and not by talking. If it continues to bother me after our session is over, then I know that it's something I need to bring up to her, so I'll email.