Nell discovers the true nature of Densi's relationship when she sees a black thong sticking out of Deeks back pocket.
"Um..."
Kensi tilts her head and raises her eyebrows. "Nell? You okay?"
The petite redhead grimaces. "Yeah..."
"Nell, seriously, are you sick? You look paler than usual."
Deeks perks up his ears and closes the file he's holding, tossing it on his desk to walk over to his friend. "Hey, you need me to take you home?" He places the back of his hand against her forehead and then her cheek, causing her to grimace again and shy away from him with her eyes wide. "What the...what?"
Observing the scene quietly from behind their desks, Sam and Callen lean forward with devilish grins on their faces.
Nell begins backing away from the bullpen slowly, avoiding eye contact with the very confused partners. Kensi and Deeks share a baffled look. "What. The Hell. Nell."
Wordlessly, Nell points her stylus in the direction of Deeks' jeans before turning and storming back up the stairs to Ops.
"What's wrong with my ass?" Deeks twists around in a circle like a dog chasing his tail.
"Noooo..." Kensi grabs a small piece of black fabric from the back pocket of his jeans. "Are you freaking kidding me?"
His eyes are wide and his mouth is wider as he stares at the thong in Kensi's hand. "I, uh..."
Kensi narrows her eyes at him, then glances at Sam and Callen. They are watching with great enthusiasm and she doesn't like it. "Nope," she says, dragging Deeks away towards the burn room.
Sam slides his chair back inconspicuously, trying to hear the conversation as it gets quieter and quieter.
"Why is this in your jeans?"
"I don't know! Shouldn't I be asking you that question? More importantly, are you wearing underwear right now? Cause wow, that would be-"
"These are not mine, dumbass."
"Well it's not mine!"
Sam pumps his fist in the air and claps his hands together with a huge grin on his face.
"Having fun yet?" Callen asks his partner with a smirk.
"Oh yes, definitely."
Callen shakes his head. "How dumb do they think we are?"
"Or more like, how dumb are they?" Sam rolls his chair back to his desk.
"So what's next? A pair of boxers sticking out of her car door?"
Sam makes a face. "I'm not touching Deeks' boxers."
Callen rolls his eyes. "So use yours."
"Yeah right, mine are way too big." Sam smiles proudly and shrugs. "Cause, you know."
"Yeah whatever you say, big guy."
Callen peers around the corner, seeing Kensi and Deeks returning to the bullpen with serious looks on their faces. They both offer tense smiles to the senior partners and head upstairs, presumably to apologize for frightening Nell now that they've had time to figure out a good story to tell her.
Sam smirks and Callen rolls his eyes as the pair mutter in unison. "Freaking idiots."