She was seen by at least a few different adults after leaving.
Yes, and she was alone each time.
It's really no mystery what happened to her. All that's missing is the identity of her killer(s() and the location of her body. The latter may never be found at this point, but we can still try our level best to figure out the former. There are suspects, though, and they are as follows:
1.) Frederick Pope - Pope, a traveling salesman, confessed to police in Ohio that he and a traveling partner named Jack Walker picked Connie up and that Walker murdered the girl and left her body in the Arizona desert. Pope went on to claim that he later killed Walker. Frederick Pope's story fell apart when police could find no record of a Jack Walker having ever existed as well as several other people Pope mentioned in his story. He then admitted to having made it up. However, a set of bones was found near Flagstaff and while they were initially ID'ed as belonging to Connie, an examination of dental records later cast doubt on this match. Little Miss X, as she became known, remains unidentified to this day.
2.) George Davies - a serial killer operating out of the Nutmeg State in the 50s, Davies was sentenced to death for the brutal murders of Gaetane Boivine and Brenda Doucette. While awaiting execution, Davies teased authorities that he had inside knowledge of Connie Smith's disappearance and, in fact, was responsible for her murder. This lead was exhausted when a search of the Naugatuck River bank where Davies claimed to have buried Connie turned up nothing. George Davies, like Frederick Pope, later changed his tune and told the police that his story about Connie was nothing more than a ploy to get a stay of execution.
3.) The Tolland County Child Killer - Now, this is a leap and one I've only made in my own head but beginning a decade later, there was a series of disappearances of young girls in Tolland County, CT. Like Connie, these were usually preteen girls who went missing while out walking or riding their bicycles in rural areas. This series is generally acknowledged to have begun with Debra Spickler in 1968 and includes Lisa White and Janice Pockett. It may have continued as late as 1979, when Deborah Quimby disappeared from Townsend, MA. There are some issues with geography but given the span of time and the fact that, all things considered, we're not talking about a huge region, it's possible that whoever was behind the Tolland/Vernon disappearances could have whetted his murderous appetite beginning with Connie Smith. Officially, no connection has been made by any of the investigating agencies.