They would have known from tire marks. Locklair is 1.2 miles and very seldom used. If the vehicle had traveled the distance those tire tracks would have been visible throughout. By all indications the vehicle turned around and dashed away left toward I-95.
Here is the other side of Locklair. You can see the street name in the dirt from Google Street View. I intentionally began on this side of Locklair and made numerous stops, including at the small fenced in area on the right. That is a local cemetery. It was locked.
Google Maps
To demonstrate that the layout has not changed since 1976 I will advance Google Street View further on the road at the south side. It is called Douglas Swamp Road. Here is Douglas Swamp Road approaching I-95. The lighter colored road is standard while crossing over freeways. Note the lack of an entrance ramp on the right side:
Here is an angled view that shows the I-95 lanes, with north on the right side. There is no way in heck this side was ever altered to remove ramps that were there in 1976:
Google Maps