Q: Weird Apple Symbol

I recently saw this weird symbol on my mac, or an indicator, whatever you call it, when you press the f1 key you get decreasing brightness, but then yesterday I don't know what I hit on my keyboard it stayed for a while, it looked like the brightness symbol, but inside there was a semi circle and the ring was much thinner, could someone please clarify what this is.

2012 MBP 13", non retina but the same generation as the retina

OSX Mavericks

2.9 dual core i7 8gb ram 750 hdd