What are the rules regarding the keeping of found property?

The laws vary from state to state. In general, to avoid being prosecuted for theft of lost or mislaid property, you must make a reasonable effort to find the owner. In most places, within a certain length of time (10 days, for example), if you cannot find the owner yourself, you must turn it in to the police. The time that they are required to hold it varies greatly from place to place and may depend on the estimated value of the property--anywhere from 60 days to a year. If, after that time, the property has not been claimed, the finder is entitled to the property.