They all have odd little qualities to them that nobody else does.
Lance, for example, doesn't like being touched. He'll lean in close, he'll tease and laugh and ask for kisses and hugs, but when someone moves to actually touch him, Lance refuses to be touched.
They don't even notice at first.
When Shiro reaches out to hold Lance's hand and Lance darts away with the excited shout that they should see that galaxy, it looks so pretty, he thinks it must just be a coincidence and his hand drops to his side, embarrassed. Lance just must not have noticed, Shiro thinks.
(Shiro misses the way that Lance's eyes flicker over to Shiro's hands and then to his own, thoughtful and regretful before he makes a fist and turns back to the window.)
Keith doesn't really notice because he mostly spars with Lance, so it makes sense for him to dodge the hits, and if Keith offers a hand to pick Lance up and Lance ignores the hand, Keith figures either Lance didn't notice or he's just being a sore loser.
(Funnily enough, Lance never comments on it. He just acts like the hand's not there, and maybe it's because if he acknowledged it, he'd feel too guilty to not take Keith's hand.)
Pidge, who isn't the fondest of being touched either, (she's alright with it, not like Lance's extremity, but she's not the type to initiate touch), so she doesn't really think it's unusual. It's not as though she carefully observes Lance's interactions with others either, so it's not as though she'd have any real reason to know.
(One time, though, while Lance is telling her to sleep and Pidge whines at him to carry her to bed, he looks as though he's reaching for her but eventually just whines that she can do it herself. She doesn't think it's important though, not then.)
Hunk can always touch Lance, with his warm hugs and his full laugh, but he has this odd habit of always telling Lance beforehand.
(It makes sense, they suppose, since Hunk has known Lance the longest.)
When it happens to Allura and Coran, they just assume it's an earthling thing.
(They don't really think about how the others never react like Lance, easily darting away with casual dismissive excuses.)
Pidge starts to really notice when she hugs Lance and he goes stiff.
His entire body freezes and he just sort of stares at her, stunned and confused and in that instant, Pidge thinks he might pull away, but instead he says awkwardly, "Um, thanks, Pidge," in a tone that very much suggests he's out of his comfort zone because. Well. This is Lance. He doesn't do awkward.
Pidge pulls away and tilts her head at him speculatively as she raises her eyebrows. "You're not the fondest of contact, are you?" She muses thoughtfully.
Lance grins and wiggles his eyebrows, though the effect is a bit ruined by how obviously relieved he is that she's not touching him anymore. "I'm fantastic at making contact in the right places."
"Ew." Pidge rolls her eyes, but she eyes him a bit differently through the day.
It's forgotten soon after, though.
Keith starts to suspect after about the millionth time that he holds out his hand to Lance and Lance smoothly flips himself up.
"Dude!" He shouts, frustration lining his words. "Just take my hand!"
Lance raises his eyebrows, and chuckles as he answers sweetly, "Aw, Keith, if you wanted to hold hands with me, you should have just said so!"
Keith turned bright red and shook his head vehemently. "No, not like that! Just... don't be such a sore loser. When I try to help you up, just accept the help!"
Lance eyes him curiously, and then cocks an eyebrow. "Aw, Keith, feeling guilty about knocking me down? So you do care."
"Yes I... stop changing the subject!"
Lance just laughs and skips away from Keith.
Then Shiro puts a hand on Lance's shoulder and he literally flips away from him and flees the room.
They sort of gape after him, before Hunk laughs awkwardly and pulls away from the group with a light, "He must have, um, remembered something important. That I should help him with. Because it's important. Yeah."
He practically flees the room after that.
And that moment is when they really start to notice.