Phasma and Hux are extremely close and have been for a long time. There is always going to be something reserved in any relationship in the FO, but they are comfortable and even intimate around each other.

+ It leads people to make assumptions
+ Phasma practicing takedowns and holdbreaks on Hux, who trusts her not to really hurt him
+ Cigarette sharing
+ Totally having each other's back and scoffing as they vent people who tried to turn them on each other into space
+ Hux turns out to be great at massages and sometimes gives Phasma one while they talk shop
+ Blunt opinions
+ Sleeping in the same bed
+ Splitting food rations. Hux barely eats so she usually avails herself of whatever she wants on his plate, etc
+ Casual touches, like wrapping a hand around Hux's waist to hold him steady in shaky transport, or Hux fixing her cape/buffing a smudge while she's debriefing
(+ If there is sex, it's not about romantic love or even sexual attraction. Just wanting to make the other one feel good/relax.)

You can toss Kylo in there, too, if you like; I mostly want intimate affection between Phasma and Hux, but I'm not opposed to them taking him apart between them or bewildering him with their total lack of personal space.