They have a hundred tricks:
1. Their one-on-one meetings are done in glass-walled rooms so no one can make accusations to impropriety (not that most people know they’re together, but it’s a level of safety Phil’s insisted on), but they also happen to be in the rooms with the smallest tables; circular, two-person tables where you have to accept the fact the only way to sit comfortably is with your knees pressed against the other person’s.
2. They always take lunch together—on the days lunch is a feasible option—and they always review work. What’s coming up, what’s just finished, what ridiculous thing has Stark done now? Phil has heard, through the impressive gossip tree that is SHIELD, that his wife (which he’s never had) left him because he never talks about anything but work. Just look at him with lunch at Fury! Yes, look at him, Phil thinks, and see the veneer and miss the coded conversation just under the surface checking in with each other, making sure they’re both having a good day.
3. Pet names are disguised by titles. “Agent,” Nick will say when he’s pleased and wants to kiss Phil in victory. “Boss,” Phil will reply to let him know the feeling is mutual.
4. Phil became Nick’s man in the field because he was—and is—the best at it. He became Nick’s husband (“Because boyfriend is a stupid goddamn word when you’re pushing forty,” Nick had explained after first saying ‘husband’ and Phil’s eyebrows rising in question) because the other option was to never have sex again in his life, given his work schedule and general devotion to the cause. At least, that’s what he tells Nick when they get a moment of absolute quiet in Nick’s unwindowed office. It’s the only way Nick will kiss him on base, and sometimes Phil just needs it.
5. They’re each other’s medical proxy because there is no one else to do it. They both approached Maria, and she informed them—in her beautifully blunt way—that she will be too busy fixing whatever the shit has broken if they’re down for the count. When injuries happen, they must always be nearby, always in contact, just in case there’s a sudden change.
6. Nick presents Phil with his SHIELD-issue taser on the day of their anniversary. It’s Nick’s birthday the day Phil threatens Stark with it, his comm channel just happening to be fully open at the time so Nick hears it crystal clear on the bridge. What a lovely set of coincidences.
7. Phil keeps a small slow cooker in his office, just big enough to make a pot of soup for two. He eats his own portion and gets Nicks to him in a coffee mug. A little food coloring, proper mincing technique, and a SHIELD capsule that changes the smells of things (one of R&Ds more whimsical inventions that sees little field use), and no one knows the difference. “You can’t survive on coffee,” the President tells Nick one day when Phil walks in with a full mug and takes his seat for the meeting. Nick just grins and takes a long sip.
8. Phil’s suits are made-to-order. It’s Nick’s tailor who pieces them together. It’s Nick himself who requests a few choice words be hidden in the hand-dyed lining his tailor also provides. Those words might be “I love you.” Or maybe they’re, “I’m glad I found you.” Or maybe it’s, “You’re the one I trust the most.” (Nick has claimed all three, but Phil never trusts him when he takes that much credit.) Phil reciprocates by putting a few choice words of his own into the lining of Nick’s duster.
9. They both still have their dogtags. They keep them in their desks. Except that Nick’s are in Phil’s, and Phil’s are in Nick’s.
10. There is a long-standing joke at the Christmas party that every agent must kiss every other agent. It’s a friendly peck on the cheek, and no one is exempt. Not even the unflappable Phil Coulson or the terrifying Director Fury. Barton was the one who started it, but the way Romanov and Hill grin every year makes it clear it wasn’t just his idea. Nick and Phil appreciate the endorsement.