I hc that Arthur knew about Bruce and Clark getting together before anyone else (except maybe J’onn) because he can sense water so acutely, he can trace the origin just by smelling it.
And one day, Bruce shows up to a League meeting “separately” from Clark, with both of them smelling freshly of Clark’s shower. Metropolis water, slightly salty, and enough of it that Bruce had clearly been showering in it for at least a couple days.
Sure enough, a few weeks later Clark walks by, and Arthur gets a whiff of Gotham Harbor water. But not just any Gotham water — the twice-filtered, faintly rusty water that flows into the Cave and Wayne Manor.
