There are a reported 50,000 foreigners living in Beijing.  But, unless you go to their expensive watering holes (Starbucks, etc.) or visit their expensive ghettos, you will hardly ever see one in 85% of the city.  Therefore it is a "I wonder what he/she is doing in this part of town?" stare.  It's the same kind of reaction that a white person gets in Harlem or a Black person in Westchester.  Don't take it personally. Smile, nod and say "Ni hao". This disarms the curiosity and always brings a big smile, a return "Ni hao" and even a "Welcome to China".