I wasn't aware of that but I'm guessing those were not single-person graves. If so, correct me.
I'll become a believer in the grave-finding IR system if it finds her body (assuming its buried).
The biggest problem for the systems is overhead cover. They still don't work well against triple-canopy jungle. However, as more of Vietnam's jungle gets cleared for farming and construction, more sites keep showing up.
At least two cases that I am aware of were single-occupant graves. One was a pilot and aircraft that augered in, the other was an Army infantryman.