They always took seats on the opposite ends of the board room at meetings never really sharing more than pleasantries and obligatory words about mission reports and briefings.
The rest of the guardians probably never even shared a moments thought about this distance. Putting it down to Hibari's cold and unfriendly personality and the fact he hardly said anything that wasn't a threat of bodily harm and that he hated crowding so distanced his self as much as possible. Or maybe they though Tsuna was still wary of the violent male. At least in Tsunas case this wasn't the reason it had been years since he had been scared of Hibari.
There were the moments when there eyes happened to meet over the chaos of the meetings. Hibari's usually emotionless eyes would burn with something other than contempt or violence. And Tsuna who had mostly, bar a few clumsy instances, grown out of the awkwardness of his teenage years. Would get a light dusting of pink over his cheeks stuttering mid sentence before averting his gaze.
Hibari some how was the most obvious of the two whether it being the cold look to anyone being overly friendly towards there boss. Or the harsh bearing of anyone one that dared to get too close. He made pretty clear to the oblivious masses without outright saying it Tsuna was his. This would have probably tipped more people off if Hibari didn't have a habit of randomly starting fights when he got bored.
Yamamoto was perfectly happy for the rest of the guardians to think he was clueless idiot. Because he was the only one who when ambling out of the board room, after a wasted afternoon, noticed the small warm smile pass between the two still seated men. He mat be clueless but least he noticed the important things.