Technician reported procedures were not followed during aircraft arrival marshaling. The technician moved a vehicle close to the aircraft to install the bypass pin and the aircraft began taxiing forward; striking the vehicle.

Narrative

The aircraft arrived to the ramp area without a radio call. We never had enough time to get set up and ready to meet the arriving aircraft properly in accordance with [procedures]. No briefing happened before this parking. As we met the aircraft; it had already stopped on the ramp; close to the air freight area. We are supposed to marshal the aircraft. As the aircraft was already stopped; my thought was a tug is coming towards to the Aircraft; I parked the van close to the nose wheel and stepped out to install the bypass pin. Next thing I heard engine sound getting loud and aircraft starting to roll again 'after plane had already come to the full stop'. There was no clearance from the ground what so ever. I immediately jumped in to the van to move it out of the way. Meanwhile the accident had already occurred. Kindly observe the second camera where the tug is waiting. None of us gave clearance for taxi to the aircraft; after it stopped.Cause was improper procedures followed.Suggestions are for better communication between cockpit and ground crew.