A BAE JETSTREAM 3100 IN CLB AT 4000 FT HAD THE CABIN DOOR WARNING LIGHT COME ON AND LEVELED OFF AT 5000 FT WHEN THE DOOR DEPARTED THE ACFT.

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ENRTE ON A FERRY FLT FROM MDW-GRR; FO AND I ONLY MEMBERS ON BOARD. GOOD WX; DAYLIGHT VMC; CLR. ACCOMPLISHED NORMAL TKOF; NO ABNORMAL INDICATIONS UNTIL APPROX 4000 FT ON CLBOUT -- DOOR WARNING CAPTION LIGHT ILLUMINATED. REQUESTED LEVELOFF AT 5000 FT TO INVESTIGATE PROB. RAN EMER CHKLIST. SENT FO TO BACK TO SEE IF ANY ABNORMAL INDICATIONS ARE VISIBLE ON DOOR INDICATORS -- IN ORDER TO DETERMINE IF FLT WAS ABLE TO CONTINUE TO GRR; OUR MAINT HEADQUARTERS. POSSIBLY JUST A FAULTY LIGHT -- ACFT DOOR CAPTION LIGHT CAN BE DEFERRED. FO RETURNS AND SAYS DOOR HANDLE IS UP; BUT DOES NOT LOOK IN SECURE POS. WE DISCUSS OPTIONS. I TELL HIM NOT TO TOUCH DOOR BECAUSE IT IS POSSIBLY UNSAFE AND IT IS NOT PLUG TYPE. HE CAUTIOUSLY RETURNS TO CHK OTHER INDICATORS (DOOR PIN VISUAL INDICATORS) AND APPARENTLY DOOR DEPARTED THE ACFT AT THAT TIME. FO RETURNS TO COCKPIT AND ADVISES ME. EMER DECLARED; WE RETURN TO MDW ARPT; VECTORS BY ZAU. PRIOR TO LNDG; FLY-BY TWR IS INITIATED TO SEE IF DOOR IS STILL HANGING DOWN WHICH MAY ADVERSELY AFFECT AIRPLANE DURING LNDG. MDW TWR SAYS THEY SEE NO DOOR -- NO OBSTRUCTIONS. VISUAL; NORMAL LNDG MADE WITHOUT FURTHER INCIDENT. DO NOT KNOW WHY WE GOT INITIAL DOOR CAPTION. I WAS LAST CREW MEMBER TO SHUT DOOR; BUT APPARENTLY WHILE I WAS IN COCKPIT WITH MY FO; A RAMP AGENT HAD OPENED AND RECLOSED THE DOOR FOR UNKNOWN REASON. I'M NOT CERTAIN IF THERE WAS A MECHANICAL PROB WITH THE DOOR CAUSING THE DOOR WARNING CAPTION TO COME ON; OR IF DOOR WAS NOT SHUT SECURELY. HOWEVER; ON GND DURING ENG START; ON TAXI; AND DURING TKOF ROLL; NO ABNORMAL DOOR INDICATIONS EXISTED. DOOR IS PRESUMED TO BE MISSING IN LAKE MICHIGAN. NO DESTRUCTION TO PERSON OR PROPERTY ON GND. SINCE IT WAS A PART 91 FERRY FLT; NO PAX ON BOARD. BOTH (2) REQUIRED CREW MEMBERS SAFE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE MAIN CABIN DOOR HAS 6 SMALL HASH MARK COLORED INDICATORS WHICH ARE ALIGNED WHEN THE DOOR IS LOCKED. THE RPTR SAID HE CLOSED AND LOCKED THE DOOR AND CHKED THE ALIGNMENT OF THE WINDOWS AND ALL WERE OK; BUT JUST MINS BEFORE DEP A RAMP AGENT OPENED THE DOOR AND ASKED IF THEY WERE READY TO GO. THE RPTR STATED THE RAMP AGENT THEN CLOSED THE DOOR AND THE DOOR WARNING LIGHT WENT OUT AND THE FLT DEPARTED. THE RPTR SAID THE ACFT WAS PRESSURIZED AND DOESN'T BELIEVE THE FO COULD HAVE MOVED THE DOOR HANDLE WHEN CHKING FOR BEING LOCKED. THE RPTR STATED THE DOOR DID NOT DAMAGE ANY OTHER STRUCTURE WHEN IT DEPARTED. THE RPTR STATES THE FAA HAS OPENED AN INVESTIGATION ON THE DOOR INCIDENT.