Buffy didn't know Elena, and despite her experiences on the network, she wasn't used to being well known, so she assumed the stranger approaching her was just a stranger. Someone who saw a woman alone in the night and felt moved to compassion and a random act of kindness.
So while she summoned up a touch of reassurance to put into the smile she directed back at the stranger, it was only the barest shred of pretense to wrap around the nothing inside. Enough for a stranger.
"I'm fine. I was just..."
The silence stretched past the point of reassurance.
"Trying to see the stars." Okay, maybe that didn't make much sense, but it should do for a concerned stranger. It was all in the tone. And the slightly brighter smile. "I've got a place to go home to."