you walk inside the building to pick something up. The building is always full of people because it is the busiest building because it is advanced from the other buildings. But when you're waiting in a long line, you hear a sound but not too loud

Looks like you're the only one still hearing that sound. You decide to go and follow the direction of the noise until you go down to the basement of the building. You look around and are surprised. Many people are imprisoned underground

...