I will cop to being a little judgey in my tone.

I have trouble believing that your motives (however unconscious) were as simple as you say. I suspect you'd have been transparent with everyone concerned from the get-go if there were zero private curiosity or stalkerish intent in seeking out your T's T.

I say this as someone who has had stalkerish impulses toward therapists and understands having intense fascination with one's T's personal life. I get it. It doesn't make you a bad person. But it is worth doing the deeply uncomfortable and often cringeworthy work of figuring out what is going on for you that you did this.

I will reiterate that keeping good boundaries is primarily the job of the therapist. Your therapist did a bad job of this and opened the door to what came next. Therapists should know that it is quite common for clients have this kind curiosity. However, regardless of what is going on for your therapist or what went on between the two therapists, I believe that it remains on you to figure out your part in this.

You do not owe this to your T, your T's T, or anyone else. You owe it to you. The next assignment in your quest for self-knowledge and understanding had been handed to you. It's yours to choose to accept it, put it away for another day or ignore it until it manifests in some other way.