核心问题
They looked in the folds of her dress, in the folds of her cloak, in the pockets, everywhere. It( = The necklace) was not to be found. [许国璋《英语》第四册，p.60]
不明白这里的 be to 具有什么样的词汇含义。我查阅了许多语法书，都说 be to 有两种含义。
A. 表示将来的一种安排：
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Their daughter is to be married soon. (CGEL) 他们的女儿马上就要结婚了。
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The Prime Minister is to visit Canada next month. (PEU) 首相定于下月访问加拿大。
B. 表示“应该”“应当”“必须”（可表示命令）：
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You are not to smoke in this room. (LDCE) 你不可在这房间里抽烟。
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The form is to be filled in and returned within three weeks. (PEU) 请于三周内将表填好并返回。
词典解答
be to + v 这个结构能够表示许多意思。其中有一个意思是一般的语法书所没有谈及的,那就是:be to 还可用于表示“可能”。例如：
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Such books are to be found in the library. 这样的书在图书馆里就能找到。
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Not a sound was to be heard. (Mis) 一点儿声音也听不到。
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Not a soul was to be seen. (Mansfield) 看不到一个人影。