What is the vetting process for asking someone to watch your stuff? Answer: nothing other than their proximity. How is that logical? Basically, you just put some rando in control of your belongings-- exactly what you're hoping to avoid. And yet, we all do it. We assume that if you sit next to someone for at least 14 seconds and they don't murder you, they are clearly qualified to take your place as guardian of valuable personal property.
In response to the ridiculousness of this social norm, let me suggest that the next time someone asks you to watch their stuff, you respond in one of the following ways: