As far as I understand people staying in the apartments are only given one front door key, which is of an unusual design.
The security of the other keys, held by staff, is one obvious consideration.
Leaving that aside the other consideration is how secure was the key held by the holiday makers?
One, or the other, of each couple would go and check on the children (and Diane Webster was instructed to return to the apartment after the alarm was raised).
There is a small detail which does not seem to get mentioned in the statements: the passing of the single key between the occupants who want to access each apartment.
In David Payne's rogatory he states:
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485 "How did it lock, and''
Reply "Yes.'
1485 "Was it secure''
Reply "Err you needed, yeah once the door was shut, you know you needed the keys to, you know to gain entry err into the apartment. Err I can't remember whether we ever had, you know whether you can deadlock it so that you could get in and out with the door open, but essentially you needed the key you know, to use, if I remember to gain access into the, err into the apartment, and you know generally it was difficult because there was, you know we'd ask about more than one key, there was the only one key to the apartment so during the day time you know
we left the key under the, the err there was a mat err outside, err you know that you wipe your feet on, and err you know that's, that's basically how we gained entry into it during the day time.'
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If the McCanns did not secure their front door key, and used a similar method to the Paynes then a potential stranger entry to the apartment would have been quick and easy.
The obvious flaw is that the McCanns did not disclose whether they did store the front door key outside the apartment.
See also "Madeleine McCann - The locks to apartment 5a"http://miscarriageofjustice.co/index.php?topic=1207.msg32131#msg32131172