I know people who were in that case. Not a fine, the taxes are just updated, and from now on they have to pay for the pool.

As for the dataset, while I don't know for certain in this particular case, we have a rather accurate database of constructions in France, that could have been used (it's publicly accessible, actually that's what OpenStreetMaps used for the building footprints here), and include pools. We also have "BDOrtho", a public database of aerial photography.

These data can be seen at https://www.geoportail.gouv.fr/carte