Originally Posted by
LondonElite
Not using the hotel is fine, but to be safe, check in in Dusseldorf, go to your room, mess up the bed, and then make your way to Cologne, returning to check out on your way to the airport in DUS. No big deal.
Most hotels will automatically check out a guest if they believe that the guest is no longer staying at the hotel. So, the scenario where you mess up the bed and leave will generally only work for 2 nights.
Another option would be to place the Do Not Disturb sign on the room for the entire length-of-stay. However, this option is not foolproof. Some hotels are required by local law to conduct a welfare check if there is no contact with the guest.
For the record: I have no knowledge about innkeeper laws in Dusseldorf. You certainly might be correct that this is "no big deal". But, unless you've done this in Dusseldorf, I'm not sure that's a given. Additionally, different hotels might have different policies relating to this issue.