This question has no perfect answer. According to the law, you can be held liable for theft if you take an item, you find and make no attempt to locate the original owner. However, I have seen thousands of detectorists unearth finds on the Florida beaches and simply keeping the items.  As I stated above, I am not sure how strictly this law is enforced, especially regarding recovering items with a metal detector. I have read news articles where someone took a found wallet from a supermarket (no idea whether the man intended to find the owner on his own), and the authorities stated he could be arrested for theft by finding. If you find something that looks modern, with any type of discernible ownership marking, you can try to locate the original owner, if you desire.