An abductor hiding in the apartment makes no sense in a number of ways
Firstly, we are told, by the McCanns themselves, that they were being 'watched' for a few days, and that the abductor had become aware of their checking regime
So why would this abductor watch the McCanns leave the apartment at 8.30pm, then wait half an hour before going into the apartment at the exact time he knew a check was due ?
Secondly, even if the abductor was stupid enough to put himself in the apartment when a check was due ( or if he was, in fact, an opportunistic abductor who had not been watching the McCanns at all, and had no idea that a check was due ) there is still the business of where to hide
The first he would have known would be Gerry sliding the patio door open and walking in. At that point, Gerry was already looking straight ahead at the children's bedroom in front of him, with it's wider than usual open door
The abductor could not have moved from anywhere else in the room to get himself behind the door without passing directly in Gerry's line of vision ( he would have to have already been standing behind the door when Gerry came in )
As to hiding in the wardrobe, even if it was accessible ( the cots ), and even if was relatively empty ( which it would have to be ) he would still, without question, make some noise doing it. The apartment was still and quiet and Gerry would almost certainly have heard someone getting in the wardrobe