A Dash 8-400 diverted to the nearest suitable airport when the flight crew received a Cargo Door warning light in addition to an advisory from the flight attendants of increasing air noise in the passenger cabin in the vicinity of the cargo door. A modest security issue ensued after landing when one passenger appeared to have simply walked away from the flight and the Captain decided their was no security risk. A decision with which several passengers took exception.

Narrative

Climbing through FL200 we received a master warning door light. After checking the doors page we discovered the cargo door was showing open. At the same time we received a call from the flight attendants who were in the galley. They reported that the noise level in the aft of the aircraft had gotten louder and asked if everything was OK. I told the flight attendants to stow the carts and take their seats for landing. We stopped the climb and started an immediate decent to 10;000 FT. The checklist was run for the master warning doors light which directed us to land unpressurised at the nearest airport. Due to our altitude ZZZ seemed to be the best choice. We declared an emergency and asked for vectors. We then called Dispatch to notify them of the diversion. The checklist was completed and the passengers briefed on the 'pressurization issue' and a safe landing was made in ZZZ.After parking at the gate I personally inspected the cargo door and; as expected; the handle was ajar. Maintenance was called to verify that no damage was done to the handle or door. After the aircraft was signed off we reloaded the passengers. At this time I was informed that one person was missing. We did depart missing this passenger which led several other passengers to be uncomfortable. After speaking with the flight attendants I made the decision to go. The missing passenger had no ability to cause a cargo door open light nor did he have the ability to force this diversion. Therefore I did not consider this a security risk.