Sylvia held her breath as she hid, relieved that Jackson understood what she was trying to do and hid himself as well. But she was still nervous, as the approaching presence seemed to be aware of her own presence. And it wasn't a surprise to her, anybody probably would have noticed the cut rope. She remained quiet as the seconds ticked by. She flinched when the door almost slammed into her, but she took in the details of the person who entered the room and shouted who was here. 

Seeing the shield and the silver spear, Sylvia realized '[i]This has to be the thief![/i]' 

So, she decided to try and knock him unconscious while she had the chance. He didn't seem to notice her there. 

So she raised her quarterstaff and tried to strike him on the head while he was distracted, and hopefully before he notices Jackson or herself. After the quarterstaff strike, she would try to sweep kick him off of his feet, just like with Dirk.