We have discussed it before on this forum but only briefly. Our current front door locks have been in use for two years. Every builder, subcontractor, rep from the promoter etc. got copies of the keys. They were laying all over the place: in open cars from painters or openly displayed on a large ring on the grass. Sometimes they were "hidden" in the flower pots on the second floor for graps for all those that didn't have their own copies.
It's imperative that you change those keys. It's an easy operation which will cost you the price of a new cylinder and some labour costs. It can be done by a locksmith, you can ask us to do it or you can do it yourself if you know how. In any case make sure that you do it before you have your furniture in. Burglars in Spain are quite familiar with these holiday resorts and they know that in the first six weeks it's easy to drive up and down with a van as it's a regular sight.
You can swap with your neighbours, but this will be false security. Burglars are not that interested in the label on the key: they just take the whole chain and try them out. It will take them a few minutes to try twenty locks and they know they won't be stopped as we are all new and can't differentiate between new neighbours fiddling with their new keychain and people with other intentions.
If you replace the lock (actually the cylinder) make sure you pick one which is hard to pick :) So don't go for the standard locksmith replacement. He'll charge you at least forty Euro's for a toy which costs less than ten Euros in a good hardware store. These locks are open within a minute. Make sure you get one which is protected against drilling and has anti pick pins which preferably are hardened. If you consider letting it out to friends or guests make sure these keys are hard to copy otherwise you'll be back where you started. People do copy them as it's handy for the rest of the holiday company or your trusted key holder considers it more convenient to give the maintenance guy a copyl
A good quality lock installed and well with five keys included and a key registration certificate will cost you less than 100 Euros. If you want even more security than described above it will set you back about 150 Euros. It doesn't matter if you give us the order or a local locksmith (there are loads of them at the costa); the main issue is that you do change them!
Your insurance won't even pay out if your lock is opened and there are no signs of breaking in!
P.S. If you have a semi-sotano remember to change the lock of the gates at the back as well!