There's a very undignified yelp from the kitchen. That voice Mike just heard sounded close, too close for it to be coming from Don in the living room. So clearly the only explanation was that Don had snuck up on him.

But when Mike spins around to confront where he thinks his cousin would be standing, he finds he's alone in the kitchen.

But...he...was just...he sounded so close.

Mike gives the room a quick once over.
Nooooope. Not in the kitchen.
So he very quietly works his way over towards the kitchen door and peers through looking for Donatello.