I knew I'd seen it before... At least you're not the only one :S
http://forums.androidcentral.com/verizon-droid-dna/266200-faded-white-dot-notification-bar.html
[Q] White dot in notification bar: what's that? Pg 2 | Galaxy Nexus > XDA
Here's an answer from one of the forums, it MIGHT be your case, but others say it's related to VoIP and most have no idea what it is, but it's not only in S4's, it's in other devices as well...
"Same here, on my Nexus4. I can make it pop up at any time by tapping on any picture to hide notification bar and buttons and then tapping again to show them back (in my case it works every time i hide and show buttons, no matter which app im using - gallery, video player, youtube, etc). That white dot then pops up. Sometimes it stays for only a fraction of a second, sometimes for half a minute, and then just disappear, whether i open the notification bar or not. My guess it's just some weird FW flaw (besides, this dot looks just like these white dots indicating hidden buttons)."
UDATE: OK, doing some digging (if you search "Faded White Circle in Notification Bar Android Jelly Bean" you'll find A TON of posts about this) it seems the most common notion is that this is a sort of 'place holder' for an app's notifications. If the app notification crashes or fails to load, the place holder won't go away until you swipe down or the system clears it. But still, it's a guess, nobody knows for sure, but it's certainly an OS bug, not S4-related.