@Carana
There appears to have been at least two sets of keys, one held in Ocean Ckub reception, and one that was available for the cleaners. In the files there is mention of the club maintenance having keys, but I am unsure if this was a third set or whether the cleaners borrowed the keys from them.
Then we have the slightly bizarre theory that Gerry Mccann offered on CNN
MORGAN: Do you know that yet? Do you know -- is there any evidence how this person came in the room?
G. MCCANN: I mean no doubt, there are a number of options. And --
MORGAN: No, actual evidence. There's nothing they could find to say this is unequivocally how this person came in?
G. MCCANN: No. I mean, it's possible they came through the window. They could have come through the patio doors, although that was in sight of where we were dining. So I think that's probably less likely. For all we know, they could have had a key, you know, lots of people stayed in that apartment over years to the front door --