When I was at lunch today a woman at the table next to mine got up to leave and she asked me to watch her bag which she left behind. Asking someone to watch your stuff based purely on proximity seems really careless. If I were that kind of person I could have stolen her bag for myself or simply looked the other way if someone came along to snatch it up. Luckily for her I'm not that kind of person but there is no way for her to know that.
That's a perfectly normal thing around here. I've often been asked to watch somebody's purse, backpack, or even their kid for a couple of minutes, if the person has to use the washroom or go to a counter to get the food order that's ready (obviously I'm talking about places like the library or food court at a mall).
I guess people operate on instinct when they ask these favors. If the person they ask seems trustworthy, they'll ask. Or sometimes it is just a matter of your being the most convenient person at hand, but even so there has to be some element of perceiving trustworthiness.
In my case the last time, it was a lady on crutches, who wanted to get a cup of coffee from the lunch kiosk in the library. I was there, waiting for my bus to pick me up, and she asked if I'd mind watching her stuff for a couple of minutes. I said yes, at least until my bus comes. So she got her coffee, my bus came, and the world still turns.
In my case, I'm not so trusting. If I'm at Walmart, for instance, and need to use the washroom, I ask the customer service staff if I could borrow a bit of space behind their counter for a few minutes and leave my stuff there while I used the washroom. They've never said no, and I've never had anything stolen. Of course I don't leave my purse behind, or anything else valuable.