DEN84IA116

Prior to taxiing at chicago, the plt had the ramp agent(r/a) check the rear airstair as the aft airstair light on the second officer's panel was 'on'. The r/a recycled the airstair & reported that it appeared closed. The light remained onso the plt referred to the minimum equip't list which required that the airstair be visually checked closed prior to fltif the warning system was inop. During taxi/takeoff the airstair handle moved leading to airstair opening about 1 ft inflight. All water was shut off in the rear of the acft to avoid possible ice build-up & the flt continued to Denver. Upon landing as the acft slowed on the rwy, the airstair opened to the full down position. The external airstair handle was found in the full up position (airstair open) after landing. The handle can be pulled down partially (not locked) & the airstair will have the appearance of being up & locked although the uplocks are not engaged. Residual hydraulic pressure could keep the airstair in the closed position long enough to give a false locked indication.