What does this notification icon that looks like an arrow with a plus sign mean?

I'm running stock Android 4.4 on my Nexus 4, and I keep seeing this weird icon that looks like a gray tag or arrow with a plus sign in it.

I don't know any app that does this, and there's never a matching notification when I pull the notification bar down. What does this mean?

Answer

This icon simply means that there are more notifications to show, but there isn't enough space left in the top status bar to show all notification icons (that's why it doesn't have a matching notification).