Tonks had left work early that night because she had decided to go to Hogwarts to see Severus as she hadn't spoken to him for a few days and she missed him. The two of them had been fairly close since his birthday when they had spoken about things which neither of them spoke about to many people and now she found herself thinking about him far more often than she thought she really should have done.
She apparated to Hogsmeade and then headed for the gates of the castle, letting herself in and then heading for the main entrance hall. Once inside, she knew she should keep herself as hidden as possible as she wasn't supposed to be there. She headed straight for the dungeon door, guessing there was a much larger chance that she could stay hidden in the shadows there as Severus often did.
Soon enough, she was in the shadows near the door to his office and so far she hadn't seen anyone. She hoped she would make it to his office but her luck simply wasn't in. She heard the voice of a boy calling someone, presumably a pet of some sort. "Trevor?" called the boy, a frown on his face, "Have you see a frog, Miss?" the boy asked Tonks and she shook her head, "No, I haven't. Sorry," she replied and the boy ran past.
She was rather relieved that he hadn't stayed long enough to realise she wasn't meant to be there. She made it the rest of the way to his office without being seen and was rather relieved to have done so. Now, she just had to wait for him.