Can your character kick-ass despite working in a Dry-Cleaners? Do they have strange, unexplained tattoos and gnarly scars littered across their bodies? Do they snap and do back-strokes on the floor as if they had lost touch with reality? Where did they get that Mk .14 EBR and how the hell do they know how to use it?
When all of these qualities fit the profile of your character, then they may or may not have been involved in a form of military or law-enforcement services and have since moved on to other things. One does not just forget these skills and may have to use them when they are needed. The reasons they have kept their past professions a secret and why they continue to do so can be for many reasons, maybe out of a sense of guilt for their on-duty actions or because they feel like it is not worth explaining. Either way, all we can do is guess.
The invention we refer to as "the Military" is a multifaceted socio-political organization that has one purpose: search and destroy. Oh, and to serve and protect their country. And provide tuition for its members. And instill discipline into those who have none. To describe the purpose of a military is a philosophical debate unto itself, but this also has to keep in mind the various divisions and branches (Army, Navy, Air Forces, Marines, etc) and their types (artillery, infantry, special forces, etc).
This isn't just something that applies to a specific military organization either. It can work under homeland intelligence services like the Central Intelligence Agency (CIA), international agencies like the International Criminal Police Organization (INTERPOL) and any other Government Agency of Fiction that has Fun with Acronyms.
- Oversaturated World: Because Classified Information is usually connected to governments and militaries, from Unfriendly Compeition - Dark Crystal, when Lyra asks Bon-Bon how her dad learned to do a sleeper hold.
"Where'd you learn how to do a sleeper hold, anyway?" "My dad taught me." "Where'd he learn it?" "That's classified."
- Walter Sobchak in The Big Lebowski makes it very clear in every scene that he has done service in Vietnam, but it is never made clear in which branch of the military he served. While the Army, the Navy, the Marine Corps and the Air Force where all involved, over half of troops involved where a part of the Army, so it is likely that Walter was in the Army. But if the original screenplay is in anyway canon, Walter did not actually serve any time in Vietnam, thus averting the premise entirely.
- Frank Martin in the Transporter movies served under an unspecific branch of the British military.
- Planet Terror: El Wray is an expert with guns and blades and is the best fighter of the whole group, but the Sheriff refuses to give him a gun because of his criminal shenanigans in the time he'd met him. Once the plot re-starts after apologizing for the missing reel, El Wray has given the sheriff his secret past (but the audience doesn't gets to hear it) and the sheriff is incredibly awed by him ("I knew your name was 'El Wray', but I didn't knew you were that 'El Wray'",) hands him a firearm and lets him carry on. His interaction with the colonel afterwards implies that Wray was part of some kind of black-ops division or at least saw military service.
- Mass Effect: Andromeda: Liam was a cop on Earth, and then joined Crisis Response, neither of which involved heavy combat training. When questioned by most party members about his surprising skill in skirmishes, he says he learned how to fight "in movies". One party member, Drack, calls "Bullshit" and demands to know what war veteran taught him how to fight. Liam admits to him that he had a mentor in Crisis Response that taught him how to fight, but that's as far as he's willing to reveal.
- Phil Cassidy from the Grand Theft Auto games is alleged to have served in the army in some games, but others say they doubt this in others. Notably, he claims to have lost his arm in combat in Grand Theft Auto 3, but Grand Theft Auto: Vice City shows he lost it due to an accident with explosives.
- It is implied that Karl Copenhagen and Quinn from Demo Reel may or may not have been members of the East German secret police and the IRA respectively. Ask and they'll deny it.
- Barbara Lake from Trollhunters knows Krav Maga, an aggression-based martial art that is commonly practiced by military personnel. Since the only visual instance of her ex-husband - James Lake Sr. - is a photograph of him in an army-green mess dress tuxedo with medals pinned to it, it is likely that she served some form of military service just like he did, or at least worked as a military M.D. at one point in her life.