It is not uncommon in Europe to see rural houses with ciscerns to catch and store rain run off for fire protection. The fire company supplies a pump and men and you have to have your own water. If they use water out of a near by cistern they will bring water to refill it and bill you for it. Let’s face it. They have homes in Europe older than our country and they are not on public utilities.
The History Channel did a show on early water supply systems in Europe. Along with a central well in the town square, each house catches the run off rain water in a cistern. Some use it for fire
fighting, and some pipe it to the central well to be filtered and piped back to the homes as drinking water. The basement and side yard cisterns also supply the water to flush toilets, mop floors, and conserve the purified drinking water supply. Some of these central distribution systems are hundreds of year old and still function very well.
like other people said, it could be an older european home and leading into a cistern, or it could be a person who is eco friendly and using rainwater to flush their toilet rather than using gallons of fresh water per flush