You need to think about what the Ocean Club was and is.
Mark Warner bought the OC around late 2006 for €1.85 million. The figure would not buy you block 5, let alone what most people think is the OC.
MW bought the communal facilities - 24hr reception, crèche above, Millennium area, Tapas area, indoor pool, adults only pool.
The apartments are owned by individuals, not the OC. Examples are Mrs McCann in 5A and Mrs Fenn above. Some owners rented out their property, via a contract with the OC. Some did not.
It is not private land in any way so the OC is not in a position to control access. Example - Mrs Murat nipping out on 3 May 2007 to get bread from Baptista before it closed. Her journey took her between blocks 5 and 6, but as the OC owned neither, it could not regulate such access.
I'm not sure if tourists booking the facilities would have been aware of that exact situation. Being surrounded by similar blocks would give the impression that they were all Mark Warner properties to those who booked via MW. Those who booked directly from owners might have had a better idea.
However the inability of MW to secure the entire area for the reasons you have posted is entirely reasonable.
It would probably have precluded them monitoring 'their' bocks using CCTV since they didn't own 'their' properties.
If they had broadly reminded or indicated a general burglary risk that might have gone some way to remind visitors ... if they had given a specific warning I don't think it would have been disregarded.
John has posted about tourists being soft targets for burglars in some areas. I believe that quite often the optimum time for burglars to strike is at the end of a holiday when holidaymakers are preparing for the journey home and have little time to spare.
There are those employed by the holiday firm who would know exactly which properties were due to be vacated.