Prior to taxiing at Chicago, the pilot 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 and reported that it appeared closed. The light remained on. So the pilot referred to the minimum equipment list which required that the airstair be visually checked closed prior to flight if the warning system was inoperative. During taxi and takeoff, the airstair handle moved leading to the airstair opening about 1 foot inflight. All water was shut off in the rear of the aircraft to avoid possible ice build-up and the flight continued to Denver. Upon landing, as the aircraft slowed on the runway, 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) and the airstair will have the appearance of being up and 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.