Although Andrew could sense when Aki was near thanks to the master-slave soul contract they had, Andrew was startled when he heard Aki's voice that he couldn't identify where it came from.
After the initial startle, Andrew noticed that the office window was open and soon, he was able to identify Aki coming out from behind one of the curtains that was billowing from the light wind that was blowing at it.
Andrew had to admit that Aki's stealth ability was extraordinary, to the point of going unnoticed even though Andrew could sense her from the contract. Fortunately, the contract existed, since it prevented Aki from harming him, since, if it did not exist, she could end Andrew's life without him noticing.
