I've noticed on Thwack this question goes back as far as the year 2004 and without any solid answers. And I read in the Orion Admin Guide where Orion uses ICMP info from the device to determine this, but I don't know how it's determined. So, since I was just asked this question, I too would be interested to know the answer myself and I'll pass it on.
Here's what I was asked:
“How does solarwinds do packet loss percentage
Is it strictly doing a ping test on an interval or what”