Disclaimer: Not mine.
You're not supposed to leave fingerprints.
Dermatoglyphics were perfected back on Earth-That-Was, before DNA mapping and genetic testing was more commonplace. The methods were simple initially; ink stamps to facilitate the comparison of geometric patterns for the purposes of identification. The old methods of collection included plastic slips for 'lifting' dusted prints, slips that would later be examined and catalogued to run against huge databases for possible matches.
That was over 500 years ago. Now those same collection slips could run detailed analysis of biochemical makeup, provide time frames based on degradation of oils left by skin, even genetic information on the individual. The scans ran instantly, feeding into the enormous interplanetary Wave system, gathering data from sources across the system to identify the individual.
Despite all the advances the most important information to be gleaned from the fingerprints was still identification.
Which is why you weren't meant to leave them.
As someone who had been in the business of taking things off folk he was all too aware that leaving behind your marks was a recipe for disaster.
So he can't touch her.
He can't run fingertips stained with gun oil across her shoulder blades, he can't slide them over her ribs and wrap them around her waist. He can't clutch at pale skin and let his fingers dig into her hip bones, can't tattoo her body with bruises shaped like large hands. His lips can't suck at her skin and his teeth can't bite into the soft flesh and leave faint impressions against her throat and collar bones.
Any criminal in the system, from the petty street thieves to the most technologically advanced cyber-crime lord knew not to leave their prints. These days prints don't have to refer to the oily finger marks left behind when pilfering loot; they can be so much more.
Electronic stamps providing evidence of hacking, providing tracers for law enforcement to follow back to you.
Specific weapons or methods of body disposal.
Patterns of behaviour indicating travel times or daily routine.
A certain type of perfume.
Anything could leave a print.
So he can't let his eyes linger on her long enough for anyone to notice, can't follow her with them as she moves across the cargo bay. He can't sit too close for fear he'll leave his own scent behind, or be saturated in the mix of antiseptic and vanilla that seems to surround her. He can't speak too much around her, can't let her pick up his speech patterns or vocabulary, and he damn sure can't go picking up on hers.
Pretentious had slipped out. Idiot.
He can't taste her, can't roll his tongue over his teeth to pick up lingering notes of warm spice and salty skin left in his mouth from encounters. He can't catalogue her scent, her taste, the sound of her voice, can't turn any tactile sensations into useful information lest he draw on it at the wrong time, in front of the wrong person.
He can't leave marks on her, can't let her leave marks on him. Fingerprints are smudged and smeared and scarred across his skin, her skin, his soul looks like a gorram crime scene.
One day someone will notice.
One day Simon or Mal or the Preacher or anyone else will let their vision shift ever so slightly and suddenly see the thick layers of fingerprints they've left on one another. The detectives will break out their magnifying glasses and microscopes, dissecting and analysing and turning the marks from relationship to evidence.
He'll be tried and hung before he can speak a word.
She nips at his bottom lip, dark eyes drawing him back down that dark path, away from his forensic reflections. He knows she knows. She rolls her eyes, smirking, shifting her hips and letting her hair trail over his shoulder as she moves her lips across his cheek, towards his ear.
"Is he scared of being caught?"
Her voice is low and calm but her cheek is a polygraph; she'll feel it if he lies.
He smirks, nuzzling into the smooth skin of her neck and refusing to care about the red marks his stubble leaves.
"Naw baby girl."
A sweet smile, pink lips and knowing eyes. "He likes fingerprints left inside and out. Likes leaving them too."
He can't help reaching for her again. There's no witty response, no verbal answer he can give her. He can't help marking her just as much as she's marked him, inside and out. He should wipe it clean, pretend nothing happened, act oblivious and let her fade into memory.
He returns to the scene of the crime, and smiles all the way.