It can mean things like: I've discovered what you've been doing. I know what your plan is.

To be on to someone means to have knowledge of someone. The same thing applies to something. Note: to be into someone means to have romantic feelings for him/her. Similarly, to be into something means to have a great interest in an activity. "The police are on to the criminal as we speak." "Good point, you're on to something here." "I think I'm in love, I'm really into this girl." "I've always been very much into painting, so I decided to study art in college."