Sometimes when I unlock my 5s using the fingerprint scanner it takes a blank picture!

I'm a bit puzzled by this one, but one occassioin when I'm using the fingerprint scanner to perform an unlock from the phone being asleep, it will snap a picture of the black screen and save it in the photos. I hear the shutter sound and that's how I know it has occured. Very randomly, but I do notice that when it does happen, it seems like it's when I hold the button down (as if I was going to call up Siri) a bit longer, but not long enough to bring Siri to my attention. It's hard to reproduce the problem on command, but it's something that will happen at least once a day.

Anyone else have this happen?