The cargo door open annunciator is one the red master warning panel on the 1900. If they took off with the annunciator light on, they would have had a red light glaring at them. If this is the case, makes one wonder how they would have reacted had they lost an engine, or had a master warning annunciator light linked to the engines go off.
If it was a system failure then fair enough. This to me smacks of complacency. When you secure the door, there is a little clear circle viewing hole to check that the door has locked. Even if it's partially locked it wouldn't have popped open until the aircraft pressurized. I was shown pictures of this, and the door is wide open just after rotation.