Little red light at top of iPhone 4?

My girlfriend has an iPhone4 running 6.0 - 6.2, last night her phone died while we were out and when she finally got the chance to charge it nothing happened. No "Need to charge" symbol and no "charging" symbol, just a small, faint red light half way in between the ear piece and top of the phone and a third of the way across the length of the ear piece. I've read that it may be a water mark indicator, which is possible. Just thought there may be another indication to why its doing it

There's no red light on any iPhone. What you saw may have been one of the sensors, the accelerometer, proximity sensor or gyroscope.