If I can't see it from where I will be, I take it with me. Tank bag, jacket, helmet, sacrificial goat, whatever, it all comes with me. I know a guy who rode from Mexico City to Patagonia and back and always left his stuff on his bike and never lost a thing, he still does it. I don't, never have and never will.
Locks like Trice has work fine.
Most people are honest, the difficult part is when a poor person is offered the temptation to take something simply so they can feed their family. Take away the temptation and everyone is the better for it. You can take away the temptation by literally taking things with you, locking them down, or hiding them in your panniers.
Panniers...yes...something I have to think about getting some day.
Metal or soft textile? Large or small? Too many decisions, I need to have a cup of coffee and look at something shiny and that will take my mind off of such details, details, details.