One way to hold this word: a spook is something that appears for a second at the edge of your eye and is gone before you can look at it properly. Everything else in the word can be traced back to that flash. The jump it gives you is the verb. The person who moves through a room the same way, seen by nobody, is the spy.
From the word’s history
English borrowed spook from Dutch spook, meaning ghost, which goes back to Middle Dutch spooc; German Spuk and Swedish spoke belong to the same family. So the ghost is not a metaphor here - it is the meaning the word arrived with, and the frightening and the spying grew from it inside English.
英文的 spook 借自荷蘭文 spook（鬼），再往前是中古荷蘭文 spooc；德文 Spuk、瑞典文 spöke 都是同一家人。所以「鬼」不是後來的比喻，而是這個字進英文時帶著的原意。「嚇人」和「間諜」是它進了英文以後才長出來的。
Wiktionary (kaikki extract) entry 'spook': Borrowed from Dutch spook (ghost), from Middle Dutch spooc; cognate with German Spuk, Swedish spöke. source_ref kaikki:3f6f69a190036ae4:782660674:19630
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