I think it's OK legally, but I guess it depends on where you live. But I don't think it's a great idea because it creates a secret that you're keeping from T. And you can't be sure how they would react - maybe they would be OK with it, or maybe not. So you hide it from them. And if it starts to bother you, now you have to make a decision whether to tell them or not, but by now you've already been doing it for a while, and you see where I'm going with this. It's important in therapy to be as open and honest as you can, and this is the opposite.

Personally, I wouldn't want anyone recording me without my permission, or at least telling me about it. I don't care if it's legal or not - it would bother me a lot that someone I knew would do that without telling me. That's just me.