He manages to keep himself from pacing by going to Shuri's coffeepot and brewing a cup, while she talks to T'Challa and then starts muttering at something on her tablet, frowning at it as she makes adjustments to whatever it is.

Bucky stares at the coffee and focuses on marshaling his arguments for T'Challa, as well as initial plans. He can't talk about the local, not to them, so how else might he plausibly have come up with this suspicion?

Those things occupy him for several minutes, long enough for the next train to arrive from the Citadel.