I cannot find a good equivalent of the Russian "Watchman's syndrome" in English. It refers to someone who thinks he's very important, but isn't.
For example, this guy has "Watchman's syndrome":
This expression is often used in Russian IT.
Learn English – Someone who thinks he’s very important, but isn’t
idiom-requestspejorative-languagesingle-word-requestsslangtranslation
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Best Answer
There is no word-for-word translation, and no single word that aptly encompasses the idea, color, and register of the Russian original. Rather, you have to step a bit back and translate your sentence as a whole.
The English equivalent to "у него синдром вахтера" is
This closely matches the Russian idiom in every respect, and is a perfectly idiomatic retort in the situation that you've quoted.