If I was your neighbour, I'd be ringing the police. @Catsos you've already said that they are looking through the windows.
Why don't you go to them and explain the situation? They may well offer better solutions than random posters like myself on the internet.
When my cat went missing, I knocked on every door in the street and the streets surrounding. Everyone I spoke to said they would check their sheds and look out for him.
It was a very stressful time, however he rolled up two weeks later, hungry and thirsty, but generally good. He was fed and brought to the vets.
Try unscrewing the cat flap.