ALTDEV ALT EXCURSION FROM ASSIGNED. UNMONITORED AUTOPLT DSCNT WHEEL SETS UP A SLOW; UNNOTICED DSCNT.

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ON ROUTINE DSCNT; WE WERE GIVEN A XING RESTR 65 MI S OF THE RICHMOND VOR AT FL230. AFTER LEVELING OFF AT FL230 I WAS DISTR BY A MINOR CABIN TEMP PROB. THE CAPT WAS WRITING DOWN THE ATIS. IT WAS AT THIS TIME THAT I HEARD THE AURAL ALT WARNING GO OFF. I LOOKED AT THE ALTIMETER AND IT SHOWED US AT FL227 AND DSNDING. I IMMEDIATELY INITIATED A CLB BACK TO FL230. ZDC DID ASK WHAT MY ALT WAS AT THAT TIME. WE WERE LEVEL AT FL230 AND I RESPONDED FL230. NOTHING MORE WAS SAID BY THE CTLR. I STILL DO NOT KNOW WHY THE AUTOPLT DID NOT MAINTAIN THE ASSIGNED ALT. I DO KNOW; HOWEVER; THAT THE CAPT AND MYSELF WERE BUSY DOING OTHER TASKS AND NOBODY WAS WATCHING THE ACFT AT THAT MOMENT. INTERESTINGLY ENOUGH WE HAD A JUMP SEAT RIDER ON BOARD AND HE DIDN'T NOTICE ANYTHING UNTIL THE ALT WARNING WENT OFF. IT WAS OBVIOUSLY A VERY GRADUAL DSCNT THAT WAS NOT NOTICED UNTIL IT WAS TOO LATE. THERE WAS NO CONFLICT WITH OTHER ACFT. SUPPLEMENTAL INFO FROM ACN 356025: AFTER A BRIEF DISCUSSION; THE CREW COULD NOT ASCERTAIN THE REASON THAT THE AUTOPLT DSNDED. MY GUESS IS THAT THE PF (FO) BUMPED THE THUMBWHEEL (VERT SPD) AS HE MADE A HDG CHANGE; STARTING A VERY GRADUAL DSCNT WHICH WENT UNNOTICED UNTIL THE AURAL ALT WARNING.