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:
1. "Ohhh, I already have been." *wink*
2. "You're going to regret this."
3. Respond by groaning. For five straight minutes. Keep groaning after they leave. And once they return.
4. Simply laugh maniacally and say, "I've got you right where I want you."
5. Agree over-eagerly, petting their luggage/belongings and cooing, "I'm going to be your new mommy now."
6. Agree, but by the time they come back, be using their stuff and have put your stuff where theirs was. Loudly announce, "NO TAKESIES BACKSIES!"
7. "I won't go down without a fight, but if I do end up taking a bullet for you, know that my family will probably sue."
8. Laugh really hard and just say "Sorry, I was thinking about the last time someone asked me to watch their stuff."
9. Cough on their stuff and then declare, "There! Now no one will take it. You're good!"