A holiday let is the renting out of a furnished home for short periods, of days or weeks, to tourists who use it as their accommodation. In the Valencia region these homes must be entered in the Generalitat’s tourism register.
For the locks it means lots of different people coming and going, keys that go missing or are not returned and the need to keep track of who can get in at any given time.
The practical approach is to change the lock cylinder when a key is lost or the season ends, keep the spare keys under control and choose cylinders that stand up to salt air. If you manage several homes, a regular locksmith who knows them is worth having. You will find it under holiday lets.