I've noticed that you cannot bring the Watchkeeper to a controlled stop. When throttles are idle, the Watchkeeper seems to develop a little bounce under 7/8 km/h which seems to keep it rolling progressively forward. Similarly, with engine off it does the same, albeit at a 3/4 km/h.
Not sure if this is linked to the PhysX wheels as posted on another bug. If it is possible too add wheel brakes that would eradicate the issue (hopefully).
To test, load in almost any map, place on runway, throttle up, throttle off, watch it bounce its way along the tarmac. It does eventually stop, but it is not predictable. Similarly engine off at any point during the process and the results are the same, but still unpredictable.