Make sure the door is always locked using your smartphone/app. How to do this can be read in this FAQ
Aparthotel Zoutelande You can put the key card(s) in the mailbox next to the check-in kiosk in the main hall.
De Grote Geere
You can leave the keys on the table in the accommodation, where you found them on arrival. Close the door of the accommodation when you leave. We will ensure that the accommodation is locked.