Yes, that her purse has not been found is maybe a huge clue in narrowing the field of possibility. –Murder or runaway, perhaps suicide, but not accident? If accident, wouldn't one or the other have shown up? Unless it was a covered up accident?
Back up thread, Sylvia’s purse came up in the context of suicide. It was speculated that if she was intent on killing herself; …‘all she had to do was ditch her purse in any sidewalk trash can’. --Yes, well Wow, what a revelation, IF that was her “intention”? Though, I think that would point toward her thinking about it long before that night. (imo)
Could she have known that there would be no publicity? -That there would be no “official” search for her? Did she act, or react in such a way that she intentionally knew how to dispose of her belongings? And never be found?
Hiring a diving team, given the terrain of Staten Island surrounded by water, bridges, and with all its internal diverse water ways would present a huge problem --where would you search without a clue? I think she would have to be running in terror to end up in a swamp by accident. Or, someone would have to know that terrain really well to dispose of a body in a swamp. Perhaps Great Kills Harbor, the Jetties, is more likely if she caught a bus home and went to the water, nearby her house? - But now, 40 years later, what are the chances that she is long gone, and out to sea?
The BF/F is the other person who is alive, and who knew Sylvia, well. I think the events of the evening suggest something passionate and spontaneous. If he didn’t kill her, did he witness, or know something? What was his view of Sylvia as her fiancé? Was she depressed in the BF/F’s opinion? –Or, unstable? Was she happy, or unhappy? Did they break up that night? He went to see her parents in person…
-If she stormed out of the car, would Sylvia have accepted a ride from a seemingly benign person, a stranger on the road? It was the 70’s. Was it a random murder? What are the chances of that? It’s possible; but maybe more possible if they were at SI Mall that she accepted a ride from an acquaintance?
ETA: Even though I wonder about the random chances, there is a known college town increase of predators at the beginning of a school year - Wagner? --Or, not hard to imagine crimes in progress at the SI Mall on a busy Saturday night, either.
--Would Sylvia run away; take no possessions, make a break from her life, her BF/F, her family, her home, her friends, her future? --And, if so, why? She wouldn’t have done it without help, or a reason… (imo)
-Still, so many unanswered questions for the beautiful Sylvia...