Seeing as Kurumi made the mistake of speaking out loud while spying on them, Yukina heard her and immediately entered a combat position. However, she could not detect where they were. Had she time to prepare, she would definitely have sent out shikigami to investigate the area first. "Everyone, please be alert. I sense enemies." she whispered, Sekkarou at the ready. 

Everything was on the line, and just because it was just the beginning it did not change anything. She had to be on top form from start to finish.