A B737 Captain reported that a ground crew member approached the aircraft after pushback without contacting them and opened the forward cargo door with the right engine running.

Narrative

We had just pushed back without headsets and delayed engine start until disconnect was confirmed by both cockpit crew members. We had started the right engine and had begun the start of the left engine. The ramp was very dark. I noticed the shadow of someone approaching the airplane at a run with his hands over his head in the hold position. He ran to the forward cargo [door] without any communication or response from the crew and opened the forward cargo door. It appeared as though he had left a personal item and was retrieving it. He opened and closed the forward cargo door with the right engine running and the left engine in the starting process. Not only was this a huge safety risk for himself but risk of damage to the engine from FOD. Also; if he had done this one minute later I could have taxied over him. This also changed our out time as a door was opened after dispatch. I can only describe the individual as a black male; medium height and skinny build. I reported this to the chief pilots office.