This is a Windows 10 thing, not a Surface thing I know, but if you’ve got a Surface you watch the Battery Icon. It’s more fun with a Surface Book though.
This morning I glanced down at the notifications area to check on the status of the battery. The Battery icon was gone, and so too was the option to customize the hidden icons.
If this happens to you, you can click in the Notifications area ^ “Show Hidden Icons” and drag the Battery icon (or any of the others) back to the Notifications area.
In this video you can see me move the Battery Icon from the Notifications Area to the Hidden Icons area, them move it around in there. Then finally I move it back to the Notifications Area.