In Japan, you cannot keep an item that you find. If you find someone else's lost property, you must turn it in to local police. Failure to do so constitutes larceny. Even if you didn't take the item from the owner, under Japanese law you can still be considered a thief. Never provoke or interfere with Japanese law enforcement people while they are performing their duties.
Japanese police have the authority to pursue suspects onto military installations.
When stopped by a Japanese police officer, you are required to give him or her your military ID card, if requested. Finally, be aware that Japanese authorities are very committed to curbing acts of violence in their society. Even first-time offenses and minor assaults on local nationals may result in lengthy prison terms. The Japanese are especially harsh on offenders who harass or mistreat women, children and the elderly. The fact that local culture and customs differ from those in our society back home sometimes tempts Americans not to take the Japanese government or its laws seriously.