This is not extra button. It’s login button which you enabled for menu in theme options- user options 

Dear Sizam,

Thanks for your response regarding the extra button. I've attached screen shots for you and also a .pdf of my theme options

The extra button/icon only appears when logged out. I've highlighted it on the screen shot for you.

It appears to be just an icon, and not a button at all. It's another login icon and it brings up the login/register popup.

I thought this was the "Additional Button in Header" option but it's not.

It only appeared today, and I'm not sure what I did to make it appear. How can I get rid of it?

Thanks

Belinda