Hey, Can You Watch My Stuff for a Sec?

I'm in the Union right now trying to read Mary Wollstonecraft, and a guy just came up to me and, nodding to where he had been sitting, asked if I'd "watch [his] stuff for a sec." I nodded and said that "yeah, sure. I'd be happy to." And at the time I meant it. But now I'm wondering, what did I just agree to?

The "stuff" in question.

This isn't the first time I've pondered this question. Pretty much every time I'm in a public space and someone asks me to watch their stuff while they go take care of some business or other, I can't help but to wonder and worry a little about what my responsibilities actually are. Naturally, it's implied that I'll watch their things keep a wary eye out for any robbers, thieves, or street youths of any sort. But if while I'm watching, as per the owners request, one of these potential malefactors comes and begins rummaging around in the "stuff" I'm watching, how am I supposed to respond? Implicit in the request to watch, is a request to prevent the stuff from coming to any harm. But what does that mean? How far should I take it? How far would or could I take it?

Ok, putting myself in the situation I imagine I'd probably say something like, "Excuse me...excuse me. Hi, yeah, I was asked to watch these those things by their owner," and then...Yeah, that's pretty much all I've got.

This is a problem. See, that's just a statement. It's not a threat or a warning. It doesn't really do anything to disincentivize any untoward actions beyond making the perpetrators aware that there are witnesses to their misdeeds. While I think social pressure can be helpful in this situation and that people are probably less likely to do bad things if they know someone is watching, is emphasizing social pressure the extent of my responsibility to the owner and his stuff? How confrontational have I committed myself to being?

This is really the crux of question for me, because, frankly, I tend to eschew confrontation...pretty much at all costs always. So am I expected to go outside my comfort zone and, I don't know, raise my voice and say something like, "Hey, leave that stuff alone! It ain't yours!"? I mean, I guess I could do that without too much trouble, but I'd really rather not. See, it seems to imply a threat of sorts, and I really doubt I'd be willing to back up such a threat if it went beyond verbal sparring. Not because I'm necessarily scared to make a scene, but I'm not really into the whole "getting into a fight" scene.

Now before you go writing me off as a coward, let's look at things a little here. Who's to say the villain here isn't going to pull out a switchblade and go all West Side Story on me. Am I willing to be a dancing, New York street gang Mercutio  all for the sake of a laptop and a backpack? Frankly, no. But should I be? I do carry a knife of my own, so it's not like I'd be completely disadvantaged, but when I inevitably win, what lengths will I have had to go to in order to achieve that victory? Will I have killed a man? I'm not like Johnny Cash here. I couldn't shoot a man in Reno, just to watch him die. Do I really need that kind of emotional baggage, just because some stranger asked me to watch his stuff? Is the next 50 years of emotional well-being worth the couple hundred buck of stuff sitting at that table over there?

I can see him coming back now, so I guess my worries are appeased for the now, but the question remains, how far am I expected to go and how seriously am I supposed to take my responsibility, to watch someone's stuff for a sec?