From The Grey House:
It was eleven o’clock. She had one hour to try and figure out a way to get a message to Vincent. Two Slayers had come to The Red Thread earlier today to set bombs. Natalia didn’t know how many; didn’t even know where. She had no idea how to send the vampire a message. It had to be done where the Slayers couldn’t see her, which meant she couldn’t slip a note to the waitress or the bartender; too obvious. She could try and signal someone, but none of the vampires she had seen the first night were here, as far as she could tell. There was one last option: write a message on the bathroom mirror.