You were twins, born together but separated at birth. One, left on Bruce's doorstep, the other; bigger twin stayed with Talia Al Ghul. To be strong to be better.

So when Damian went with his father, after being told about his twin. He was curious. A bit irritated. But curious as to what this twin was like. He listened to his father Bruce, call for his twin and watched the entry to the Batcave intently.