Ruling on a woman covering her face when praying because non-mahram men may pass by
Question

Is it obligatory to cover the face when praying if there are non-mahram men present or they may pass by, as happens in the Haram? Or is it o.k. to uncover the face?.
Praise be to Allah.Shaykh Saalih al-Fawzaan said: 
When a woman is praying her whole body is ‘awrah and she must cover all of her body except her face, if there are no non-mahram men present. 
If she is alone or with men who are her mahrams, then she may uncover her face when praying. 
But if she is in the presence of non-mahram men, then she should cover her face when praying and at other times, because the face is ‘awrah. 
Fatawah al-Mar’ah al-Muslim, 1/315. 
See also the answer to question no. 21803 and 1046. 
And Allah knows best.