Yes, I dissociate during sessions (and outside of them too). It's happening less often as my anxiety gets under better control.

My T is very good at spotting when I'm not all there, but I also hide it really well, so it's taken her a while to figure it out with me. It's very difficult to tell T when it's happening, but when I can tell her, it's always a good thing. When I tell T that I'm dissociating, she helps me get grounded and keeps checking to make sure that I stay present.

Perhaps you could talk to your T about what you can say or do to let her know that you're dissociating, prior to it actually happening. Then, if you have a plan in place, it will be easier to let T know what's going on.