Oblivion never ceases to amaze me.

Recently, my character joined DB and few days later I was on my way to leave Cheydinhal and was walking from abandoned house to the east gate. Unfamiliar woman passes me and says "Yes, sister?" It could only be dark sister - I look at her and don't recognize her - she's wearing causal clothes and a bow. Then I look once again and realize it's a Bosmer woman from the sanctuary, Taelandril or sth similar.

I was surprised, I never encountered DB members outside of the sanctuary so far, posing as normal residents in casual clothes. Feels kinda magical when you discover something new after playing the game for so long.
Do you know if any other DB members leave the sanctuary and can be encountered outside?

No, she is the only one.