I just skimmed through the responses on this thread. I think DY chopped the issue the best here:
“There is no clear rule for these things. It depends on who is person is. If you’re not comfortable, there’s probably a good reason for that. Even if you are, maybe you shouldn’t be.”
The issue,(as I understand it), isn’t about halacha. It’s about comfort levels. When it comes to tznius, a lot of things are not black and white issues but rather are about comfort levels. Girls need to trust their feelings when it comes to tznius and shouldn’t be made to feel bad when they are uncomfortable with a particular situation.
Personally, I would have knocked on the door, but I’m not LB. It’s also possible that I’m wrong.