Answer:

You can turn it in at the counter of the Service des objets trouvés (Lost-and-Found Office, 5, rue des Glacis-de-Rive – 1207 Geneva) during opening hours, or deposit it in the secure letter box that is accessible 24/7 and located at the same address. You can also leave any found object at any police station. If you have found the object in a vehicle belonging to the city’s public transit system (TPG), you can also leave it with the driver or an authorized inspector.