Matt is Frank's lawyer and/or friend, and eventually Frank notices that Matt is getting injured all the time. He knows that Matt's excuses are bullshit and decides to start lowkey following him around to catch who ever it is thats beating up an innocent blind guy. He's really good at blending in and is confident that even if Matt could see he wouldn't know that Frank is following (stalking) him.
He doesn't know that Matt can hear his heartbeat and knows exactly what he's doing.