What does it mean to say: "I'm onto you"? -- please read details?

A friend of mine whose 1st language is not English - just like me. He met a girl at a bar, and it seemed like they connected as she asked for his number. Later that night, she text him that phrase, he thought she was drunk and it was a typo for "into" and text back "I'm into you too" but she text "that's not what I meant" and changed the subject. What are the all the possible meanings of that phrase? And what are the differences?