T'Challa happened to be walking past the tree Diana was sitting in. He wasn't doing anything in particular though. Like her, he was only exploring certain areas of the city out of curiosity and to get a general memorization of his environment. It never hurts, in case of an emergency, to know your way around.

He almost passes by the tree completely considering he was a little deep in thought himself. But he does happen to notice Diana in the tree. His senses were far above average so even the smallest rustle of the leaves or the change of scent in the air would give it away. The Wakandan king looks up into the tree, happening to spot the Amazon. It doesn't take a genius to figure out she has something on her mind.

"... You seem troubled."