A Columbia 400 pilot reports a door warning shortly after takeoff. An attempt to relatch the door resulted in the door departing aircraft. An emergency was declared and the flight returned to their departure airport.

Narrative

1) Preflight airplane. 2) Start engine. 3) Listen to ATIS. 4) Radio ground on information and intentions. 5) Follow instructions for taxi and takeoff. 6) Taxi to Runway. Turn on switch for door seals. No warning that doors were open or ajar. 7) Climbout at 800 FPM using autopilot. 8) At 2500 FT; hear audible warning 'Door Open.' 9) Review situation and noted that doors were fully closed and pins were set in door frames. 10) Audible warning increases in repetitions. 11) Momentarily takeoff headset to listen for any noise of air leaks in either door -- none noted. 12) Turn off 'sealed' door switch. 13) Reset door latch; hoping to turn off sensor. 14) Door opens immediately and tears off from airplane in seconds! 15) Radio Approach for emergency landing. 16) Land airplane and check for visible damage -- only missing door noted. 17) Check locks on luggage door. 18) Noted one lock not properly engaged. A) Never open door latch unless in water or moving slowly on ground. B) Land airplane immediately when possible issues arise that warrant further attention.