A Company Technician reported an unattended aircraft parked on the ramp overnight rolled forward into a truck due to the chocks being removed and the parking brake not set.

Narrative

Aircraft X arrived at ZZZ on Day 0. The aircraft underwent an A-Check that night and was scheduled to depart ZZZ1 on the morning of Day 1. The aircraft had been parked overnight on remote parking Spot XX and the A-Check was accomplished there. At approximately XA:00 on Day 1 Ramp Personnel arrived at the aircraft to move it to its departure gate. Ramp Personnel removed the chocks from the aircraft then left the area. Upon their return to the aircraft; they discovered that the aircraft had rolled forward approximately 15 ft. The left engine had come to rest on an airstair truck. The airstair truck had been in use that night during the servicing of the aircraft. The truck had been pulled away from the aircraft in accordance with TSA unattended aircraft policy. It was determined that the individual that removed the chocks was working alone. There was no tug connected to the aircraft and there was no brake rider. The chocks had been removed without determining the status of the parking brake. This action was in violation of the Ramp Service Manual 'CAUTION:CHOCK REMOVAL REQUIRES PERMISSION FROM THE BRAKE RIDER'.Suggestion - Refamiliarize move personnel with the Manual XX-XX-XX Aircraft Tow and Brake Procedures. Emphasize that it is not safe to assume the status of the parking brake.