Thing about a smuggler's ship is there's all kinds of places to hide.

And that ship makes little creeks and groans all the time. In fact, Kirk has probably noticed the weird sighs that came while he was investigating the Millennium Falcon and completely disregarded them. Apart from confirming that yes, this ship is such a pile that it makes noise even while sitting on the ground.

But those little noises, could have possibly covered the occasional boot step. Han is also, damn good at hiding, because of certain unfortunate events, which, for now, will remain undisclosed. He creeps up to within a few meters of the intruder and then, literally, slams his warn black boot into the durasteel. The metal, can at times, be clanged against hard enough to make a person's ears ring, though he didn't go that hard.

"What are you doing on my ship?"