He tries to pretend he didn't see it.
First thing that morning, Callen didn't see it. Or at least his brain didn't process that he saw it. It might be that his brain was trying to save him from that image.
When he does see it for the first time, he thinks it kind of looks like she got stung by a bee. He almost asks her, but decides it's less important than the case.
A little later, he thinks that there is no way in hell its a bee sting. But maybe another bug bite? A bruise? He thinks of this millions of possibilities. He's not sure why he's thinking about it so intensely. He knows Kensi is a big girl and that she can take care of herself. She doesn't need him looking out for her.
The thought that it was a hickey didn't even cross his mind until that night, when the case was closed and they all went out to get drinks. He'd seen the budding of something starting between Kensi and Deeks. He was one of the people who could honestly care less, as long as he didn't have to see it at work and it didn't affect their ability to keep each other alive. He'd had a notion that things went to the next level between them, but he never saw it.
It was at the bar that things clicked for him.
Because at the bar, the partners became lovers. They were with their work friends, their family. But here was a place where they knew they were allowed to be them. Kensi was tied to Deeks' hip, and anyone from miles and miles away could tell that they were in fact, in love. He didn't mind seeing them be a little more... flirty, in this setting. It was kind of a relief, to be honest.
It was when he saw them kiss that the connection between the red mark on her neck and Deeks was made. A hicky. Duh. How could he be so stupid? He's even got a girlfriend (and a really good one too, one that he really really likes) and yet a hicky didn't even come into his mind.
Although he did have that flash image of Deeks putting it there, which totally made him shudder a little bit, he was proud of the agents. He was proud that they got their act together, that they made each other happy and that they could keep it from affecting their work. He really wanted the best for them. They deserved it.
It was later that night when Kensi and Deeks were lying in bed that Deeks pushed up her chin to examine the hicky.
"Do you think anyone noticed?" he asked softly, tracing his finger around it.
"No one said anything about it, so I guess not," she replied.
He pressed a little tiny kiss to the irritated skin. "Sorry about this," he apologized. "Couldn't help myself."
"Just leave it somewhere a little more... private... next time."
"Come on. You can't tell me you didn't get a thrill out of this. Walking around all day hoping no one would notice."
"I didn't say that I didn't like it, but..."
"I'm teasing Kens. I love you," h" offers.
"I love you too."