There isn't one particular area - many live near the park in Bella Vista/Mirador Sur, some live in Arroyo Hondo, Naco and Piantini, others in Gazcue/Ciudad Nueva/Zona Colonial - the main generalisation would be that on the whole they tend to stay west of the Ozama.
Quite a few years in the Cap and I can say hand on heart the only time I bump into expats is outside the bars on El conde down ZC. Never see them in the malls, restaurants, driving around, walking around, the cinema. I think there is something going on under the city, they go from apartment to Casino via underground tunnels.
I SEE,"fOREIGNERS" in Supermercados quite often.I usually say "Hola",just to see where they are "from"!
That's how I met "HillBilly",in Supermercado National Santiago.I was admiring the size of his "Banana"!He invited me to his house on the spot!
I live in Arroyo Hondo,SD.
I don't think they are in the "Casinos" a lot,maybe because I am not!