Two little computer icon on my menu bar
Unless I am mistaken, this double screen icon is the screen-sharing indicator. This is what it looks like for me in Sierra.



I can't tell you why its blinking or disappearing, but if you can click on it, you should have the option to see which IP address is connected, and an option to disconnect them. If your computer is managed by someone/some entity (work or school), I would go to their IT office and ask them why this is appearing or if they know anything about it.

If you own and are the only person in control of your computer, then I would suggest you change your User password immediately. Then log out, log back in with the new password. If the icon reappears, then comment below this answer as it might be a virus.