Unfortunately, picking up the keys and returning them turned out to be a problem. On my way out I left the keys on the postbox #4 inside the building on the right wall for those who enter the building. I can remember it vividly because I was carrying heavy luggage and I had to put it all on the floor to drop the key inside the box. When the host told me the keys were not there I read the instructions again and realized I had misunderstood: there was another postbox #4 right across from the opposite wall. However, anyone can do wrong when it comes to left and right, specially when you're full with package rushing to the train station. What might have happened is that the owner of the postbox #4 I dropped the key kept it. For the incovenience I am really sorry.
Nevertheless, I wish the host could take into account that they weren't really helpful when I first got in touch about my late time of arrival and how to get into the building. When I rentend the room months in advance it took them more than one week to answer me how they could give me the keys. After 2 unanswered messages in a row the short and vague response I got was that "someone would be there for us". When I double checked this information asking for a phone to call, just in case, they did not answer me back, at all. In the end, very last minute instructions were given, including secret codes to get the keys in a different location from the apartment. Despite that incovenience, I managed to get the keys. But it was not as easy as having someone there as they first told me there would be. I wouldn't recommend Schon Apartments, although the flat was really nice.