It must be a background service that needs to be in the notification shade so it has priority and doesn't get killed. Could be a battery saver app or something like Tasker, a lot of them have an option for "no icon" but it actually still takes up a space in the notification shade even with no actual notification. I would look in background services under apps in settings and see if you have any apps installed that would need to take up one of those spaces. Hope that helps

 

I have an app called 1-tap Quickbar that does this. It lets you put a row of shortcuts/toggles in your notif. drop-down, but adds an icon to the top bar. The icon can be changed to transparent in the settings.
Apparently, this is just the way it is with android- anything that gets added to the drop-down shade must have an icon at the top.