As a resident in Spain, taxes must be paid from the total income. In general, when you are not a resident and you own a property, you pay the property tax and the non-resident tax. If you are a resident, you pay like any other Spaniard, the taxes from the total income. Hence, once a year, an income tax return is made by the tax authority. However, the tax aspect for senior citizens is unfavorable because the tax paid from their pension is definitely higher in Spain than in Germany.
It is possible to present proof to the tax authority, but in practice, these are only used occasionally. It can happen that the tax authority verifies the power and water supply of the apartments or houses. Therefore, it can research if the property has been inhabited during the entire year. It is your decision if you want to be a resident or a non-resident.