Even before the saboteur entered the farm, Yona was already observing him from afar. Thanks to the upgraded barrier, even those people who have yet to enter the farm can already be detected, especially if those people have ulterior motives that can directly harm the farm.
Since emotions are easily captured through magic, even the slightest change can be detected by it and with Yona being very good at controlling the magic that was entrusted to her by Julia, she has gotten so good at manipulating and controlling it that one might say that even if she didn't make the magic itself, can be said that she have expanded the usage of it that she even incorporate it to some of her magic spells especially interrogation spells. Yona has Julia to thank, as without her making this barrier and explaining the intricacies, perhaps she would not even understand how it totally works.
