The first time Beckett notices, it's early December and she's finishing up paperwork on their latest case, the murderer safely locked up behind bars a couple hours ago. Her head feels heavy, her eyes drooping a little from the strain of the screen. Her shift had ended forty minutes ago but she knows herself; if she doesn't do it now, something else will come along tomorrow and then the administrative — hate-inspiring and painstaking — part of the job will never get done.
Castle is sitting in his chair next to her desk, playing some game on his phone as usual. He used to run away at the first signs of her opening up the reporting programme, but lately, he's been better, even helping her at times. Unsurprisingly, he's great at remembering the smallest details. Beckett has to be honest that when she's in the heat of the moment, her note-taking isn't always as meticulous as she'd like it to be. It's another way their partnership seems to be a perfect match.
He's in full concentration, swiping left, right, up and down, and then pauses for a minute to let an ad play. As he continues onto the next level of his game, he starts tapping with his foot, a strangely familiar rhythm to it. A couple moments later, he starts humming along with the tapping and Beckett recognizes it as a popular Christmas song.
She smiles a little to herself, shakes her head, and continues writing her report.
The second time it happens, they're in the car, waiting for a suspect to exit the office building across the street so they can intercept him. He had put the radio on five minutes into the stake-out. She had promptly turned it off again, saying that it would hinder their concentration on the job ahead.
He had complied but only for a couple minutes. He had turned the radio back on and, at the poignant look thrown his way, turned the volume almost all the way down. At least this way he wouldn't go too stir-crazy and blow their cover, she had thought, so she allowed it.
They sit there, listening to the quiet hum of some Christmas music station when a particular song comes on and Castle starts to quietly sing along. The words are a little mumbled under his breath but he is hitting every note otherwise.
She turns her head slightly to look at him while also still keeping an eye on the building's entrance. He is looking out the window at the people passing by. She supposes coming up with secret background stories for each of them. He ones tried her to join in on his favourite stake-out pastime but to no avail.
He probably doesn't even notice he's singing along right now. She doesn't say a thing, though, just relaxes into her seat and enjoys the smooth low tones coming from her partner.
As the holidays come closer, Castle starts to sing Christmas music more and more often. And louder. What used to be silent and under his breath has become for the entire precinct to hear. Especially at times like this, past normal business hours and with most detectives already on their way home.
Except for the four people in Beckett's team. They had caught a new case half an hour before their end of the shift, which is why they are currently in the conference room, going over phone records and bank transactions of the victim.
Each of them are deep into the numbers when Castle starts humming again. Beckett sighs, mourning the comfortable work silence they had going on. It's a catchy tune and it isn't long until Esposito and then Ryan join in. It takes them about an hour longer than it should have to go through all the records tonight.
Beckett knocks on the loft door. She knows she's early for the Castles' Annual Holiday Bash but she has come right from her shift. At this hour, the roads are too busy and it would have taken her hours to go home to change and drive all the way back across town. So she had changed at the precinct and made her way to the party from there.
Alexis opens the door with a "happy Christmas" and lets her in. The entire floor has been decorated with streamers, tinsel, and what she guesses are the younger Castle's old artwork.
Castle is moving around the giant tree in the centre of the room, putting the finishing touches on his masterpiece (she has heard all about how to achieve the perfect tree trimming this past week). The sound system is playing a soft song as Castle sings along, equally as soft.
She hangs up her coat on its usual hook and smooths down her red dress. As she crosses the living room, the holiday spirit must possess her because she starts to sing along to the music as well.
Castle startles at the sound and turns around, a big smile on his face.