Nothing prevented her from being able to sneak up on a window. She expected that the glass would've been enchanted to hide what was actually inside. To not find even the barest attempt at concealment bothered Hermione immensely. Did someone want her or anyone else to stumble upon the house to see what was happening within? Any further questions racing through her mind were temporarily silenced by the entrance of her brother-in-law to the room she was spying on. Ginny crossed in front of the window moments later, seemingly unbothered by an attempted murderer in her house. If the two of them were present in a house together, it was only natural to assume that William Wood was inside as well.