I often think similar things, then realise they must know each other well enough to recognize each other even from a distance.
For example I can't see long distance very well and need glasses, so while I can see peoples faces up close, more than about as little as 10+ yards I start to struggle to define faces, so the amount of people that have tried to get my attention from across school yards or opposite side of the street I have always had difficulty. But then I realised that after spending a lot of time with my friends I could figure out who they were from the clothes they wore to the way they walk, so one can only assume that it is similar when talking about fully covered muslim women.
And just throwing this out here on slightly unrelated topic; while I am fine with people having religious beliefs and customs, burkas freak me out -because- I can't see their faces. After so long struggling with faces it just makes me uncomfortable when I am not able to define a face now. So that's just me putting that out there, nothing against the gals xD