In my experience, I've found that people tend to not bother collections of iron as it's usually difficult to steal without noticing, and the returns for the risk aren't that great. That being said, I would not leave any metals with real value out and accessible. I'm talking about items like copper and catalytic converters, which are almost guaranteed to disappear, and also things like aluminum rims and batteries. I run a small metal recycling facility and have known many people personally who have had these types of items stolen from them. With metal prices up and the economy down, I hear more and more of these stories every day. To summarize: light iron/steel SHOULD be okay to leave on a pickup, anything more valuable be aware that you are taking a risk.