Hey, cool, I love it when people/cyclists watch out for one another. This past summer, I noticed that someone had missed locking their bike up by putting the U-lcok through a pole, but not getting it aound any part of the frame. I always doulb-echeck myself, but this person hadn't and there bike was left completely unlocked and anyone could have ridden it away. This happened in front of a large store that had a PA system and so I went to the front desk and told the person there what had happened and they announced it over the PA ("Person with the green Peaugout, please come to the front desk") and they re-locked it properly.
I hope that the day my bike gets stolen or when I screw up like this person had, that some kind-hearted soul is able to help me out.