A DORNIER IN CRUISE AT FL280 EXPERIENCES A DOOR WARNING. ACFT DSNDED PER PROCS AND MAINT ADVISED. NO LOGBOOK ENTRY MADE ON INCIDENT.

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IN BRIEF; AFTER LEVELING OFF AT FL280; THE FO AND I CONFIGURED THE ACFT FOR CRUISE FLT AND SETTLED IN FOR THE REMAINDER OF THE FLT TO ZZZ. APPROX 30-40 MI FROM BOS; WE WERE ALERTED BY A TRIPLE CHIME AND MASTER WARNING THAT THE MAIN CABIN DOOR WAS NOT SECURE. LOOKING OUT OF THE CAPT'S L SIDE WINDOW; I COULD SEE THAT THE MAIN CABIN DOOR WAS NOT OPEN. THAT; COUPLED WITH THE LACK OF 'CABIN ALT' WARNING; LED ME TO BELIEVE THAT THE DOOR WAS IN FACT CLOSED. AT THIS POINT; WE RAN THE REQUIRED QRH CHKLIST; FOLLOWED BY MY XFERRING CTL TO THE FO SO THAT I COULD CONTACT MAINT CTL. I ALERTED THE FLT ATTENDANT TO THE POTENTIAL PROB AND REQUESTED THAT SHE LOOK; NOT TOUCH; THE MAIN CABIN DOOR TO SEE IF IT APPEARED TO BE CLOSED AND LOCKED. SHE INDICATED TO ME THAT IT APPEARED TO BE NORMAL. AFTER TRYING TO RAISE COMMERCIAL RADIO ON 3 DIFFERENT FREQS; I SECURED A RADIO-TO-PHONE RELAY WITH COMPANY OPS. WHILE SPEAKING WITH COMPANY/MAINT CTL; THE FO; AT MY DIRECTION; BEGAN A DSCNT TO A LOWER ALT. WHILE AWAITING A RESPONSE FROM A RADIO EXCHANGE; THE INDICATIONS OF THE PROB DISAPPEARED. I PASSED THE INFO ALONG TO MAINT CTL AND BEGAN TO DISCUSS THE INCIDENT WITH THE FO AND AT THE SAME TIME REVIEW THE SYS PAGES FOR INFO AS TO THE POSSIBLE CAUSES OF THE ANOMALY. THE CABIN PRESSURE CTL SELECTOR PANEL PAGE SHOWED THAT THE ACFT WAS PRESSURIZING AT A SLIGHTLY HIGHER THAN NORMAL RATE. I REACHED UP TO POINT AT THE CABIN PRESSURE CTL SELECTOR PANEL WHILE SPEAKING WITH THE FO. AS I WAS POINTING TO THE PANEL; I TAPPED IT WITH THE TIP OF MY INDEX FINGER AND THE TREND ON THE SYS PAGE REVERSED AND SETTLED INTO NORMAL PARAMETERS. CONFERRING WITH MY CREW; IT WAS CONCLUDED WE COULD FIND NO REASON FOR THE NOW ABSENT; YET SPURIOUS INDICATIONS AND WE AGREED TO CONTINUE THE FLT WITH HEIGHTENED VIGILANCE. WE LANDED IN ZZZ WITHOUT FURTHER INCIDENT. AFTER OFFLOADING THE PAX; I CALLED MAINT CTL VIA CELL PHONE AND DISCUSSED THE ANOMALY. WE AGREED THAT THIS ANOMALY; FOR LACK OF A BETTER DESCRIPTION; WAS PROBABLY ONE OF THE MANY; 'COMPUTER REBOOT REQUIRED/STRAY ELECTRON' PROBS THAT HAVE ALWAYS INFECTED THESE AIRPLANES. IN MY CONVERSATION WITH MAINT CTL; I INDICATED THAT I WOULD DISCUSS THIS ANOMALY WITH THE CREW PICKING UP THE ACFT. ABOUT THIS TIME; THE NEW CREW ARRIVED AND I DISCUSSED THE SIT WITH THE NEW CAPT. I DISCUSSED THE POSSIBLE NEED FOR A MAINT ENTRY IN THE ACFT LOG AND AFTER THE NEW CAPT CALLED MAINT CTL AND CONFERRED WITH THEM; WE ALL AGREED THAT IN THE ABSENCE OF A PROB OR ANY INDICATION THEREOF; THAT A LOGBOOK ENTRY WAS NOT WARRANTED.