That's why I don't allow my cats outside. I know it's different in Germany.

Tell your Mom to check the perimeter of her house, every hidey hole there is, including any and all crawl spaces. A cat doesn't stray far from their food source. That's what I did when Lil Eli fell out of my livingroom window in Michigan. Since he was semi-feral and I lived on a VERY busy street, I figured he was a gonner. But I didn't give up. I tried trapping for 5 days. On the 6th day, I came home from work and found him sitting in the middle of the back yard. I was so elated, I ran towards him and he immediately ran into the entranceway to the basement, in a dirty, spider web infested crawl space. I was never so glad to see him in my whole life!!!

He's now enjoying his life on my friend's son's dairy farm in the UP of MI (upper peninsula-very cold in the winter) in a heated barn with cows, and chickens and is an excellent mouser. I miss him but know he's happy.