We just go about our day, assuming that everyone will hold themselves to the same level of honesty that we have.
College is a great example of trust.
I trust the people in my hallway. We're all very friendly to each other, and I couldn't imagine any of them stealing from me. Even so, I don't understand why people don't lock their doors when they go out to class. Maybe out floormates would say something to the thief, but the odds are better that either no one will see it or nothing will be said for fear of being rude. Imagine coming back from class to find your whole room was cleaned out.