He really should’ve noticed that something was terribly, terribly wrong when he saw her hand on his knee.
But, to be fair, Ted had a lot of other stuff on his mind-- important stuff, like realizing that he’s been kidnapped by murderous time-traveling crazies who are really into the whole retro thing, and that the old man who he’s been visiting every day is now suddenly a lot less old but still really weird about home security-- so it was reasonable that he didn’t notice how close Audrey and the Once-ler were sitting on the couch, how his hand would sometimes find itself in her hair while he was talking. It was a stressful day.
It was the little things that got him suspicious, though he made up excuses for each one. She would put her head on his shoulder as he knit-- Audrey was a touchy-feely kind of girl, she’d do that to anybody. He built a garden for her in the backyard-- so what, Ted had helped with that too; he’d dug up the land while they were off getting water for 45 minutes. One day he woke up to find her eating breakfast dressed in one of his shirts-- maybe all her clothes were in the laundry! He had an explanation for every little thing.
Until the day he saw them kissing behind the house. He couldn’t explain away that one.
One day, as he walked home from school with Audrey, he finally asked.
“Hey, Audrey? Are you and Mr. Once-ler, y’know. Dating or something?”
She stopped suddenly on the sidewalk and looked down-- always down-- at him, a little smile on her face. The kind of smile you give a child when he’s asked a stupidly obvious question. “Yeah, I guess you could call it that.”
They continued walking.
She didn’t laugh, at least, but that didn’t make the words any easier to hear. “I’ll tell you when you’re older.”