A day off school is no big deal but that, plus not answering her calls or texts, and also blowing off hanging out in the afternoon with no notice? That is not typical Billy Batson behavior. Steph considers going straight to his apartment and see if he's there--maybe he's still here, maybe he just fell in the shower and broke a hip or something--but then decides to call Korra to see if Billy showed up for their training session. If, for whatever reason, Billy's fine but just avoiding her...well, Steph can deal with that. Pissed off boyfriend is much preferable to injured or dead or disappeared boyfriend.

When Korra hasn't seen him either, Steph is officially worried. She's waiting at the bottom of the building for Korra to arrive, so they can go check his apartment together. If he has disappeared into thin air...Steph doesn't want to have to find that out alone.