What is this down arrow icon for?

It might be an indicator for a backup service or antivirus like Norton that checks and shows status icons for what files have been scanned or backed up.

Did this icon used to be something else and then changed?  It could be that the registry controlling which icon to use is corrupted.  

I think you're on to something on both counts:

It occurred in a synced folder with Google Drive, but I checked with them and it isn't their icon. I have Sync Center disabled, so Drive is the only program running a sync to this folder. The reason I'm investigating it is because older versions of my files reappeared in my folder (and in some cases saved over the current files) when the icon appeared - so I suspect in part it is a Google issue as well.

It could have to do with some sort of corrupted registry though too. When I moused over each file after the arrow icon appeared, the icon "refreshed" and turned back to the Google Drive check mark icon. Do you know how to identify the registry and repair it?

Thanks

Honestly no, at least not that I could immediately point to from memory.  But I can help the investigation.  This page may help quite a bit: https://productforums.google.com/forum/#!topic/drive/_zN32ac5n18

It describes a bit of how the wrong icon (or no icon) can occur, and where the registry keeps the icon overlay identifiers.

Be careful not to get the registry in your eyes ;)

Thanks for those. I'll dig into them further to find a solution.

In the meantime, the top link you provided leads to a page that recognizes the down arrow "overlay" as what appears when a file is ready to be written to a disc. This answers the main question of the post - something many Microsoft Answer Desk reps didn't know.

...though it shows that it is definitely the wrong icon to appear in the situation that it did for me, so it looks like I've got more troubleshooting to do.

I've had to troubleshoot these before myself... it is unfortunately a bit tedious usually if you reduce how many programs are trying to use overlays, it resolves it.  One other tool that might help is Microsoft AutoRuns (bing it for the direct download page).  If you know of MSCONFIG's options to disable auto-startup apps, this is like that on steroids, but works similarly.  You can uncheck to disable even individual shell overlay icon handlers.  For example these three entries in my screenshot are the handlers that show an icon in the various statuses a one-drive synced file might be in.

Unfortunately this only identifies the DLL file, not the actual icon used (that part is in the registry), but since you can just uncheck the box to disable it, you can probably use this to easily find out which one is causing the drawing of that arrow.