57, 58 Plymouth? Could be a Chevy too. Probably stolen. Back then people left their keys in the ignition.
I know a guy that bought a car he couldn't afford. This was in 2003 and I think the payments were north of $700/month. He, fortunately, bought gap insurance because he put down like $100. I think the car was north of 40k. I don't recall exactly.
He decided he wanted to quit, but he knew the car was going to get repossessed and dinged on his credit.
He drove out to a quarry near Philedelphia and gave it the heave-ho, walked a couple miles and had his buddy pick him up. Called the police in Philly, said his car was stolen. It was found 6 months later when they drained the quarry. Insurance covered the entire loss. As a note, I'm not the buddy and I've not spoken to the guy for at least 15 years.
So a long story short, these cars could be crime cars, stolen or insurance fraud.