Superman smiled when he heard the kid approaching, the distinct sounds his body made making it easy for Kal to tell who was approaching him. It was almost impossible to distinguish it from a human's heartbeat, but as soon as he got confirmation Mark happened to be Invincible it he couldn't just... unhear it.

"No, but I'm sure he would eventually have. I decide it was time to take a very smart young man's advice and stop making things more difficult for myself."