In your case where you could see her at an event such as the one you describe, I would still keep contact to a minumum. Do you attend these with a friend or on your own?

If you were with a group and your t was also, I think it is appropriate to either smile and say hello, but not go out of your way to see your T and make further conversation. Focus on who you are with and let her focus on who she is with. That eliminates the awkwardness of people asking who this person is (on your end and the Ts end) and just keeps things separate as they should be. In the office, your T is a close confidante. But outside the office, your T is an acquaintance. I know this is hard to reconcile but that is the reality. Keeping it friendly but simple is polite but respectful of your T's need for a life separate from her work.

If you are both solo at the event, you could venture a little further and say hello with some small talk. I'd also keep it short and simple though, as the same scenario applies - she is an acquaintance out of the therapy room and that is the way it is supposed to be. Of course there is no need to avoid her altogether, but to make a concerted effort to socialize could make your T uncomfortable and you do need to respect that.

That's just my take on it, so I hope it doesn't seem harsh. Using your therapy as a place to go over this is a great idea, and you may learn a lot from your T. I could be way off the mark and she might have a different perspective. Either way I think it is an important way to learn about the boundaries of other people and how to adhere to them even if we don't agree with them. This is one of the things T's have (or should have) expertise in, so use this as an opportunity to learn.