My mom is Vanessa's moms age and she uses "ping" like a verb. She could be referring to anything from texting, finding me via an app, etc. So I think it depends on the field you work in, and perhaps even your age. We live in Central Mass if that helps. Until more information comes out, "ping" could mean a lot of things, but it is not information from her cell phone carrier. I'm assuming that the police are in contact with her carrier.
She would have had to leave Princeton by 4pm a the latest in order to make her bus in Worcester, so they were probably aiming to leave between 3:30-3:45 to play it safe. I have taken that bus route many times, and it's one that you would want to show up with plenty of time for. The NYC buses, especially on the weekend, tend to be crowded and if you don't want to get a terrible seat next to someone that smells, you get in the habit of showing up early. I think they probably started getting worried sooner than 3 because she would have needed to shower after running.