Two girls

The other day, two girls went to the shop close by. These two girls often go to the shop. let's call the girls one Mary and the other one Bridget. As they were walking back towards me,  I saw Mary had some hidden stuff under her jacket. What went through my head was, did she steal them? why was she hiding them? What would you have done in my position? I felt a duty towards Mary to teach her right from wrong, I would have liked to stop this habit at an earlier age if so. But I was thinking, what if her mum yells at me and tell me to mind my own business. So I said and did nothing. I saw that Bridget wasn't happy to go with her the next day, so later on when I saw her alone, I asked her why did she hid the stuff she bought? there was no response.   Just letting you know what you would have done in my position?