Explain the significance of the sinister blackboard message that Daru finds in the last paragraph. Who might have written it and for what reason?

The writing on the board was bitter irony for Daru. He was being watched but the watchers did not know that Daru had given the Arab prisoner a choice of freedom. The people watching could have been any number of people rebelling against French imperialism. I think the point was that Daru will never be accepted into Arab culture regardless of what he does or does not do.