Respuesta :
He became suspicious of a prowler because it would not be normal to knock on your own hotel room door before entering.
A burglar might do this to see if anyone was in the room. If there was nor response they could easily enter the room and take what they wanted.
Nesbo realized that if the man really believed that was his hotel room he would have had a room key. Since he was knocking, it is likely that he was merely checking whether the room was occupied or not. If it was empty, he could break in and take what he wanted.