The presentation focuses on the daily work practices of staff in Belgian immigration detention centres. Though I consider all categories of staff in my research, I will focus particularly on "social staff - by which I basically mean all non-security staff that interacts with detainees. Having to work in a secure setting with unknown detainees, I argue that the principal activity of staff is to observe individuals and groups of detainees. Through daily meetings, where all this information is shared, the unknown detainee become a "knowable subject" and decisions are taken on how to handle. Thus, despite the official discourse of humane detention, detainees are permanently considered as liabilities and "social" workers turned into risk managers.