Detention centers can be leftovers from colonialism because we consider the people in them second class citizens. We do not even grant them the rights of prisoners of war at Guantanamo Bay. You could argue that this is because we had "taken over" Iraq and considered them our people.
Post colonialists argue that the attitudes of colonialism still live. They argue that racism and imperialism still inform the actions of colonial nations.
I assume that you are talking about detention centers like Guantanamo. If so, a postcolonialist might point out that such centers are based on the idea that the colonial nations can treat citizens of other (mostly colonized) nations in whatever way they wish. They can hold them indefinitely without having to justify their detention. They can (importantly) abuse them in ways that belittle their culture (Abu Ghraib is an example of this). In these ways, the colonial powers show that they still see the ex-colonies as inferior to them and believe that they can do as they wish with the people of those countries.
In this way, a postcolonialist would argue that detention centers prove that colonialist attitudes are alive today.
or am i reading in to it too much?
Would it be right in asuming that detention centres such as guantanomo are also a way of regaining some of the power lost through de-colonisation?