I just had Sierra installed almost 2 days ago. I have a late 2015 iMac. Happened yesterday that I got a pop up informing me to call a certain number (Which I did not of course) Installed Malwarebytes and ran numerous virus checks using Bit Defender and every scan came up clean. What I did notice yesterday is when I start up my Mac the apple logo lights up red (around the entire logo) for about 10 seconds but then takes me to my log in screen.
I thought that maybe this had something to do with the pop up that I received prompting me not to shut down my computer and call the number provided.
Everything is still running smoothly for me. No freeze ups, shut downs, rebooting issues or anything of the like. Still running virus checks and malware checks every several hours with nothing to be found.
Not sure if it is part of a security boot up check that is part of Sierra or what, but I had never noticed it before either. Will continue to monitor it over the next several days. While I am sure it is nothing, if it bugs me enough or I notice any strange behavior I will just reinstall the o/s to see if that changes anything.
Update Dec 4th 2016
So I did some research on some apple forums and a lot of people are experiencing the "Red" apple logo at startup. Apple is aware of the issue and are stating it is a cosmetic issue that is being worked on and there should be an update to get rid of it soon. Many of these people tried reinstalling Sierra but the issue is still present. It has been reported it really does not affect anything else