Part of what makes a good doctor is having an excellent imagination. It's why he loathes transporter beams - he can come up with a million scenarios in which things go horribly, messily wrong without trying hard. Thus, it really doesn't take much for him to come up with possible causes of a scar like that. Adding in the possible anatomy underneath adds a new level of horror - sure, she surely isn't baseline human, but she's at the very least humanoid, and even Vulcans haven't strayed too far away from the common plan. Underneath that scar are at least three major organs, five major vessels including the descending aorta and vena cava, a major chunk of the nervous system... the only thing it doesn't cross is any weight-supporting bone, but that's a near thing.

"Smells wonderful." He says appreciatively, coming up behind her, his hands sliding over her waist. And if his left hand presses a little more firmly than necessary, well. He's a doctor. He's always a doctor, he can't turn that off. He has to have a little reassurance, or his imagination will go nuts.