You are right, Gendarmeria is supposed to patrol only the borders. However, they have the reputation of being the least corrupt police force, and for quite some time now they have been assigned other duties. For instance, Puerto Madero is so safe because it is patrolled by Gendarmeria, who don't sell nor tolerate "liberated zones" in the area. And San Isidro's infamous villa La Cava now has Gendarmes guarding the entrances. I drive past it often - there's a plaza in front which used to be deserted, as it was not safe. Since Gendarmeria set up a post there, it is full of children playing, and adults sitting around gossiping.