If you are in Germany and have lost something, you can make inquiries at the Lost and Found office. Maybe someone has handed the item in. There are official Lost and Found offices in all major cities. Transport companies also have such offices for objects left behind in buses or trains. The service is free. For valuable objects, it's common to offer a finder's reward.
If you have found something yourself, you are obligated to turn it in if it is worth more than 10 euros. You do not necessarily have to go to the Lost and Found office, however. If you find a wallet in a bus, for instance, you can hand it in to the driver. At agencies, museums or swimming pools, you can also hand it in to the office. The owner will probably be pleased, especially if it's a valuable object.