I think the flower just builds some sort of a virtual reality simulation that 'updates' itself to what it 'sees'. It saw gray workers get hired, and so just integrated that information into the simulation. It probably learned of their personalities and relations through observation and conversations. Yume's power doesn't work in the simulation/dream simply because the flower isn't omniscient - it has to 'witness' it. So either her nose was never set off (so the case was basically very safe for them), or it got triggered when they talk loudly about the power, so they didn't.