hidden factories

There are buildings with no roads that lead up to them. Generally made of red brick, with high, narrow windows. We have one on the outskirts of our small town. I don't know how anyone applies to work there. I heard you get a letter, with a contract to work for 2, 3, or 5 years. 

It's not a 9 to 5 job, whatever it is. The people walk out there, go inside, and they stay. They don't seem to ever come out the whole time. After their years are up, they walk back in to town. Most of them leave town and never come back. Those who stay keep to themselves, and don't seem unhappy, but none of them ever seem to work anywhere else again.