The knocking T

A situation has recently developed in my sessions and I'm just wondering what others would do.

My T works in an office with 5 other Ts. They each have their separate rooms, there's a small kitchen and a waiting area and bathroom. My T's room is the one right next to the entrance. It's an old apartment building, the walls aren't that thick and the doors are just flimsy wooden doors not meant to really keep anyone out or in.

For the last three or four weeks, another T has started knocking on the door during my session. He apparently wants to say good bye to my T, the conversation always goes something like *knockknock*, my T saying "yes?" in a rather reserved voice, the other T (does not open the door) saying "good bye Ts first name", then he says good bye too and then you can hear the main apartment door. Yesterday, he even knocked three times in total. He apparently first didn't hear my T, then knocked again which my T didn't seem to notice, and then after a minute or so again, where he then explained he didn't hear him the first time.

I'd be fine with this happening like once. Or once a year or something. But it's not like there's thick walls, I know for a fact that he could hear us talk if he'd pay attention. I can hear both other sessions (if I wanted to, I could probably understand what was talked about) and my T talking on the phone with a closed door before my session. And we are usually talking, maybe there's a short silence or something, but we're not just sitting there for 50 minutes not saying a thing. Also, my T leaves his door ajar when he's not in a session and the door is clearly closed. I think the other T might also be checking whether he's the last to leave the office, so he'd know whether to lock it, but my session has been at the same time for 5+ years, by now it should be known that usually my T is still around.

So, I've been thinking about what to do. I could go down the most polite route and tell my T that it bothers me and he'd probably have a word with the other T. The other idea I had was to yell my Ts usual response before my T can get his first yes in. I think that might be kind of funny, though maybe it'd come across as rude?

Do any of you guys have a good idea? I know I could just ask my T, but I kind of feel like embarrassing the other T a little bit, he should really know better than to knock every week...