red square around apple logo at startup
I installed Sierra (on my iMac (late 2015, 2.8 GHz) on 09/17/16. After installation, midway through the boot screen, a red square flashes very quickly around the apple logo. I started my Mac up in safe mode and the red square does not appear. I ran first aid in disc utility and it reveals no issues. Today I installed Sierra 10.12.1 and the issue is still there. Is this a sierra bug?
iMac (21.5-inch, Late 2015)