3
sigh
After wondering about this for several days, I just realized what it was.
If you do a "Find All", you're provided with a list of search hits. You can select an item from that list. That marker is for the line matching your currently selected search results item.
indeed. I had that same question once some years ago. – Frank Thomas – 2016-11-17T19:55:12.303
In prior versions of VS, you could use Find-in-files to "Mark all Occurrences" of a find result with these icons. – Frank Thomas – 2016-11-17T20:04:57.790