She had been down at the lake when she had heard the two sixth years talking about something they were keeping up in the tower. They had seemed like they wanted the whole thing quiet, which had immediately peaked Hilary's interest.
So, she had followed them. She had not intention of turning them in, in the event that they were doing something forbidden. She was just curious as to what they were keeping in the tower.
She saw on of them turn, looking to see if anyone was behind him. She quickly ducked to hide behind a suit of armour, before continuing after them.