The new Balearics tourism law will establish fines of up to 40,000 euros for the illegal renting of holiday apartments (the rental of houses, villas and so on is dealt with differently, and most of this accommodation can be legally rented out and marketed, so long as it is registered). The law, it is hoped, will clarify the situation in respect of apartments. What appears to now be set to become law is that apartments cannot be "commercialised", i.e. established as a business and marketed, for instance, via internet websites, but that apartments can be rented out for a week so long as there no "commercialising" going on (e.g. advertising the property), this being a new element that gives a time limit that had previously not existed. The mere fact of marketing an apartment for tourist let would seem to mean liability, therefore, to a fine, regardless of whether the property is let or not (or this is an interpretation). Whether this does clarify the situation is questionable. Anyway, the law is due to come before the regional parliament at the end of this month, so hopefully all will become clear.
See for example: El Mundo