Diane looks so young for her age. She would have been a senior in high school.
The photo of her in pigtails was her sophomore year in high school. She doesn't appear at all in the 1973 yearbook (photos would have been taken in the fall)
Okla Dr (upper middle class area) is just a little over 1 mile from her high school (WT Woodson HS). The locations of her school and home are between Fairfax and Annandale, VA
Google Maps
It sounds like Diane had a pre-arranged meeting with someone who she didn't want her parents to see. Most likely a young man. It sounds like she was either eloping or running away, but it's seems a bit odd that her friends didn't mention recognizing the car or the person she left with. Was the person a total stranger to them? How would a girl like Diane, leading a sheltered life in the days before cell phones and personal computers, manage to develop such a close relationship with a complete stranger?
There's no mention of her missing at Newspapers.com. Local LE must have assumed she was a runaway, not realizing she was in danger.
Some speculation: She had been involved in some very lucrative real estate transactions recently, assuming due to her inheritance from her uncle. Could that have created some jealousy from another family member - a cousin, another uncle or aunt, etc?
Alternatively, she could have made a new acquaintance at her bank or local real estate agency that handled her real estate transactions. Perhaps someone several years older than her who paid her a lot of flattering attention when he realized she had financial assets. Old enough (maybe mid 20's to 30's) that her parents would disapprove of her dating. Perhaps someone who was married?
She may have led a sheltered life and trusted someone whom she thought she was romantically involved. Since she never accessed her money, she may have met a tragic end.
Odd, though, not seeing any news articles or public appeals from her family to help find her. Obviously , someone close to her wants her to be found, considering her profile has been updated in recent years with new information.
JMO, ICBW.