file out

MURDER at the Tokyo Lawn Tennis ClubでKawamura警部は一連の殺人事件の犯人で、しかも警部の家族までも襲わさせた、テニスクラブのpresidentである、元駐米大使のMorimotoと対決すべく、Tennis Clubの理事会が終わるのを待っていました。
Within a minute or two, the conference room door at the rear of the lounge area opened and the fully dressed board members began to file out.
"file out" は初めて見る表現ですが、理事会が終わった場面なので、理事がぞろぞろと会議室のドアから出てきているのを示しているに違いありません。　この"file" も以前取り上げた表現 "indian file"（a line of persons or things ranged one behind the other
　Ref. http://blog.alc.co.jp/blog/3302827/archive/2010/07/5 ） と関係があると推測できます。
・Wiktionary: To exit in line (in a single file).: People filed out in a single line to get through the narrow door.
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: [for a line of people] to move out of something or some place. The people filed quietly out of the theater. They filed out at the end.
・Vocabulary.com: march out, in a file: Look at the schoolboys as they file out of school.: After the lights come up, everyone files out and rushes home to write about how awesome/terrible next season looks.
ぞろぞろと出てくる場面を視覚的にに想像できる表現ですね。