I've never seen it as I should protect their bag that they left. I don't think I'm even asked to. I think they just bring the attention to themselves and the bag so I know what they look like so that I will react if someone else starts opening their bag. Also I think what they are asking me is if I am going to be there a while, until they come back.

I think they only expect me to say "Hey that is not your bag!" And they expect the other person to be scared enough to just leave. I don't think they expect me to physically protect their bag or put myself in danger.

That said, it is weird they do trust me. But in a weird way I can understand that. There are a lot of hidden contracts in social situations. Connections between people can happen even if they only last for a few minutes. Also they have seen you and know what you look like if you take off with something in their bag so I guess they trust that will scare you straight.

Of course this trust is an illusion. But also, most people are not thieves so statistically it should go OK. In a way it comforts me that strangers do trust me. That not all humans think other people are bad.