The "enter" sound he heard was not heard verbally but through morse code. He had heard the sounds so often that it directly translated to the word in his head. What it actually sounded like was a series of short and long knocks. Morse code was mainly used during the war and it was less common in this city to be used as communication. Of course the spies and assassins who trained to kill knew it but an untrained ear could not catch the nuances.
That is why, for their security system it made the most sense for communication. It was essentially their version of "siri"but without words. On the other hand, if someone were to try and break through their security but make a mistake, the warnings would also be communicated through code. Currently it was set up to engage an alarm but also communicate a word for level of danger. The semantics were debatable but it worked well for them thus far.
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