I remember reading that, it's here

La cara b del alquiler en Madrid: cuatro familias latinoamericanas en un sótano de 40 metros por 2.500 euros al mes | Madrid | EL PAÍS (elpais.com)

Unfortunately this is very common across European cities now and will only get worse. It used to be students sharing a property and now it's multiple families and criminals have found another easy way to make money. The problem is that the situation can quickly turn and people who want to rent out their property looking for a profit might end up with Okupas. There is a lot of frustration and even though renting out to tourists might seem profitable, same story if it's on the web advertised as holiday accommodation. Okupas will look online and can see when it's empty. This is also why many sellers decide not to show the property address on Idealista anymore.

It doesn't look like there are any tips - plus don't forget you need to do a tax return every 3 months by the looks of it.

Makes me wonder just how many non-EU owners just don't declare at all.