I had something similar happen when I worked as a bagger for a local grocery store during my college days. (Scissors being one of their ad icons). I'd finished bagging up an order, and the lady was putting them into the cart herself. After she'd left, she'd apparently left one of the bags behind. By this time, I was bagging the next lady's order. Just as she was leaving, I noticed the bag. I asked her if it was hers, she took one quick look and said 'Yes!', took it, and off she went. I thought nothing of it, until the first lady comes back looking for it, describing its exact contents.
Thankfully, the management was cool about it, realized it was an honest mistake and I didn't get reamed out for it. But I really felt bad about it afterwards.