Found items and the Second Precept

There was a crumbled up dollar bill on the stairs when I was coming out of work today. I looked around and didn't see anyone who might have dropped it. I picked it up and walked away. Pretty soon I went back and laid it back on the steps again. I realized I really didn't know how the 2nd precept, "I undertake the training rule to abstain from taking what is not given" should be applied to found items which don't have an apparent owner. There are lots of examples: *wild berries in the woods *wild flowers in a vacant lot *something interesting that washes up on the shore *something that's been left on the side of the road *money lying on the ground with no one anywhere around *an apparently abandoned item in a desolate area I hadn't really thought about this before, but as I was walking away with that dollar I actually felt kind of guilty. So I'd like to know found items should be considered for the future. Thanks!