Red box on Finder icon in Dock Please see this image:



I don't know what's going on with that icon, but it has been persistent for the past week. It became like that when I forced the shutdown on my Mac because it hanged up. I've just upgraded to El Capitan when it happened.

I've tried the following:


Reboot
Relaunch Finder
Quit Finder and Dock on Activity Terminal
killall Dock


Nothing worked so far.

Update:

As per user24601's suggestion, I've tried the following:


Turned on magnification. It persists on zoom. Please see image:





Tried to change position. It still persists:
I ended up reporting it as deeeeeeekun suggested. Some of the suggested workaround in the comments were also suggested to me. This is the solution they gave that worked:

1) cd getconf DARWIN_USER_CACHE_DIR

2) attach com.apple.dock.iconcache to radar

3) rm com.apple.dock.iconcache; killall Dock

I'll update the answer when they give me the reason why the redbox happened.