Just because that wasn't them doesn't mean they aren't there. Someday you might want to do some sort of sampling of bees to see if any show up. Seeing them on bees on comb is often just luck. And doesn't tell you much.
Then again, ignorance is bliss. Bliss is good, sometimes.
On bees, I usually see them on the thorax. I don't usually see the underside of bees, but efmesch is probably correct. Because, I believe that the underside of the abdomen is where it is easiest for varroa to feed.