B737 Captain reported a near collision between an unmanned fuel truck and the aircraft at the gate; while conducting a preflight inspection. The fuel truck driver was able to stop the vehicle before any injury or damage occurred.

Narrative

While conducting the exterior preflight inspection; I witnessed a near collision between the fuel truck and the aircraft. While I was walking the parameter of the right wing; the fuel truck appeared outboard of the right wing; moving toward the terminal; then circling; making a u-turn toward the leading edge of the right wing. The next thing I noticed; the fuel truck continued moving toward the wing; but without the driver onboard. The truck appeared to still be in gear; moving approximately 3 miles per hour; now heading underneath the wing. The truck cleared the underside of the wing and just missed contacting the engine. It was now heading toward the aircraft fuselage; aft of the wing. The driver began running alongside the truck; attempting to jump into the driver's seat. Approximately 6 feet prior to impacting the fuselage; the driver was able to stop the truck's motion. He then proceeded to circle the wing again and fuel the aircraft. Since I witnessed the entire event and was able to confirm there was no impact between the fuel truck and the aircraft; I completed the remaining portion of the exterior preflight inspection. Once returning to the flight deck; I called the Chief Pilot to report the near mishap. The Chief Pilot stated she would contact personnel in ZZZ to ensure appropriate action taken.