Locking the box is the only way to go, like @jademartinez said. It's wild how some people treat a toolbox like a free rental service and return everything covered in mystery gunk. You gotta wonder if they even see the difference between a socket wrench and a soup ladle. Honestly, that lock is less about security and more about sending a clear message to stop treating your stuff like a community condiment.
My old 10mm socket came back looking like it dredged a fryer vat. I found my good needle-nose pliers with what I hope was just barbecue sauce in the hinges. At this point, I'm pretty sure my coworker thinks my toolbox is a community kitchen drawer and every tool is the gross spatula nobody wants to wash. The only thing he ever leaves clean is his conscience.