A B737-300 FLT CREW AT THE GATE IN DTW WAS WITHIN SECONDS OF PUSHING BACK WHEN THE JETWAY AGENT OPENED THE FORWARD BOARDING DOOR WITHOUT NOTICE.

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WHILE OPERATING FROM DTW WE WERE INVOLVED IN AN INCIDENT; WHICH HAD A HIGH POTENTIAL FOR SERIOUS INJURY OR DAMAGE TO THE ACFT OR JETWAY. WE WERE WORKING A LAST MIN MAINT ISSUED (APU FAILED TO START). NO FURTHER STATION ASSISTANCE WAS REQUIRED AND WE CLOSED ALL DOORS AND HAD THE JETBRIDGE REMOVED TO EXPEDITE DEP WHILE AWAITING OUR NEW FPF AND MAINT RELEASE. DOCUMENTS WERE RECEIVED AND I WAS ON THE HEADSET WITH THE PUSHBACK CREW (I HAD BEEN ON HEADSET FOR ABOUT 5 MINS). WE HAD ENG #1 STARTED AND ALL EXTERNAL EQUIP REQUIRED FOR THE START REMOVED. THE CALL WAS MADE FOR PUSH CLRNC AND WERE ABOUT TO PUSH WHEN THE MASTER CAUTION AND FORWARD ENTRY DOOR LIGHT ILLUMINATED. I LOOKED OUT THE L WINDOW AND NOTICED THE JETBRIDGE HAD RETURNED AND THE CABIN DOOR WAS OPEN. UPON REACHING THE PURSER ON THE INTERPHONE; HE INFORMED ME WE HAD BOARDED 3 PAX. AT NO TIME WAS I; MY FO OR THE TUG CREW MADE AWARE OF THE RETURN OF THE JETBRIDGE. IN A RADIO DEBRIEF WITH STATION OPS THEY INFORMED ME THEY HAD BEEN TRYING UNSUCCESSFULLY TO REACH US VIA VHF. I GENTLY REMINDED THEM OF BOTH ACARS AND THE PUSH CREW. A SIMPLE WAVE FROM THE OPS DOOR; LOCATED NEARBY; MAY HAVE WORKED ALSO. IN A DEBRIEF WITH THE PURSER; HE INFORMED ME THAT HE WAS JUST AS SURPRISED AS WE WERE; AND FEARED THE AGENT MIGHT OPEN THE DOOR WHILE STILL ARMED. HE WAS IN THE PROCESS OF CALLING ME WHEN THE DOOR OPENED. THERE WERE NO INJURIES OR DAMAGES; BUT I FELT THE POTENTIAL WAS SO HIGH FOR EITHER OR BOTH THAT A SAFETY RPT WAS WARRANTED.