B737 Captain reported a cabin door handle moved open during takeoff roll.

Narrative

During takeoff; immediately after stating 'Check Thrust'; the Master Caution illuminated with a 'Doors' light and 'Aft Entry' light on the overhead. I immediately rejected the takeoff with airspeed approximately 54 kias. All lights extinguished at about the same time I brought the thrust levers to idle. We coordinated with tower; exited the runway; and completed the rejected takeoff checklist. After communicating with the flight attendants; we determined that the L2 door handle was not completely seated. One of the flight attendants saw the handle move when we began our take off roll and he then immediately pushed the handle down. He was convinced that the handle was now properly seated. We then coordinated with both Dispatch and Maintenance Control; and made an ELB (Electronic Log Book) entry. Based off of all available information; both our entire crew and Maintenance Control were comfortable with us proceeding to ZZZ without any further action. After coordinating with Tower; we took off and proceeded uneventfully to ZZZ.