Who does Percy overhear talking in Chapter 2?

In Chapter Two, Percy overhears Mr. Brunner talking to Grover.

I was three steps from the door handle when I heard voices inside the office. Mr. Brunner asked a question. A voice that was definitely Grover's said "... worried about Percy, sir."

I froze.

I'm not usually an eavesdropper, but I dare you to try not listening if you hear your best friend talking about you to an adult.