Snape knew that Voldemort had sent you to spy on him, but you didn't know that Snape would later on catch you and attack you. You were watching Snape from a window as you some type of animal to not make him suspicious of a person watching him. You saw him leave his office, then you heard a twig snap. Once you turned around, you were tied up with a rope, struggling to be freed, Snape walked up to you

"You think I wouldn't notice your little activity?"

He held a dagger up your throat