I think that they most likely used the sense of smell to
triangulate the Indians location.
Given that its night and the original poster said that sight was
not one of the senses employed, we have to assume that its pitch
black and no one can see anything.
So how could they smell out the Indians? It's not because they as a
race 'smell', rather it was because there were so many of them and
they were in a huddle that they sweated (when you huddle in a
closed group, you heat each other up faster than if you were just
standing alone). The sweat was evaporated and carried by the cool
night breeze, which in turn gave the Indians location away.
The Indians couldn't smell the Chinese because they weren't huddled
and/or there were not enough people to generate that kind of heat
to make them sweat.
As for the radar, all the dead bodies masked the movement of the
Chinese who used the bodies as camouflage moving from one to the
next, like barrier to barrier. (Remember radar does not detect
heat).
