It's the little things. The way Arthur sits with his legs carefully aimed away from Eames. The way Eames can't stop touching his own lips when Arthur's around. The way Arthur's jaw clenches to keep from smiling when he sees Eames. The way Eames says "darling" only to Arthur, although he flirts outrageously with everyone.
Without these little signs, these tiny moments, no one would know. But of course they all know, always have, and every time Eames joins them for a job, the rest of the team carefully fades away at the end of the day, letting Arthur and Eames get a cab together. The next day, everyone avoids certain topics: the almost-concealed hickey on Eames's neck, the ever-so-slightly mussed look of Arthur's normally impeccable hair, love.
At the end of every job, as Eames goes home to Mombasa and Arthur prepares to start working again, the rest of the team screw up their courage and vaguely mention how much they like Eames, or miss him, or how well Arthur and Eames work together — in a strictly professional sense, of course. Nine times out of ten, Arthur's face goes perfectly blank and he suddenly has urgent business elsewhere.
But every once in a while, he nods and agrees. Every once in a while, he lets them see the way he and Eames are tied together, the way they move in relation to each other, the almost imperceptible moments when they're communicating on a different wavelength than the others. Those little things give the rest of the team hope that someday Eames won't go back to Mombasa, and Arthur won't throw himself into the next job.