Today, I noticed a image I downloaded earlier (one of two) has a large gray circle at the right-hand edge of the Name column in Finder:
Note that the other image made at almost the same time doesn't have it.
This is not a tag. When adding the gray tag to the file, it looks like this, with a second small gray circle.
The large gray circle is something different. What is it?
This is macOS Sierra, 10.12.4, on a 2013 model Macbook Pro. Last night, when the image was made, neither had a gray circle. It's appeared sometime in the last 24 hours.
Edit: the file is located in Downloads, and (edited from original) was actually a downloaded file from Chrome. I do not have iCloud sync enabled for files. The gray circle was not present last night or earlier today; in fact since I posted the question, the circle has disappeared, reappeared, and disappeared again - only for this file and no others.