I guess in a way, it is Trespassing. But I think of it this way; Would you call the cops for someone picking up your dog crap in your yard for their garden?
Well, I think of it this way, it wasn't your dog crap or your brass. It was a valuable commodity that you trespassed on someone else's private property to steal. No different than walking in someone's home and taking the loose change off their dresser. Hard to believe you were ever a LEO if you have no better understanding of the law than this.