He was quite perceptive, and a lot of people who could be considered a threat who were within his range of detection would become someone who would be paid attention to, and he would try to remember them to see if they appeared again later.
If they happened to appear again later, then he would increase his vigilance toward them even further.
Ash did not know if he had seen her when she found him or when she had been following him, so she could not afford to appear near him again, as it would only result in a struggle between them if he reacted violently toward her.
If a conflict began, then things could become dangerous for her, even if she was careful, as it would not be beneficial for such a thing to take place.
If things happened in a way that could draw the attention of others, then she could not do what she had been tasked to do, since there would be many witnesses and it would be very difficult to get away with it even if she happened to succeed.
