It's not as if Captain Solo is an expert at not being snuck up on. If she's reasonably trying to hide the sound of her footfalls among the ambient noises of the beach, then he won't notice the sneaking - besides him feeling a bit distracted by this thoughts.

"Maybe the party isn't why I came." He says with a shrug. "Maybe I changed my mind."

He doesn't have to justify why he approached and then walked away, does he? Though he does notice the happy and that makes him hold off from saying anything else. He can't remember her seeming this happy before, as a matter of fact.