She claimed to have overheard the competitors discussing their strategy at the cafe.

overheard 是动词 overhear 的过去分词形式。overhear 由前缀 over-（表示"超过、额外"）和动词 hear（听）组成，字面意思是"在无意中听到"或"偶然听到"，通常指无意间听到别人的谈话。