Hide and Seek
When she heard from Kelly about Garrus' discovery after returning from Tuchanka, she went straight to him. This was something that he had been following him since the day she brought him aboard the Normandy.
Finding Sidonis had been his goal.
At the battery, he explained his plan to her. Shepard didn't like it. She didn't approve of his plan to kill Sidonis. He may have good reason to, but he knew better than that. Still, Shepard decided not to question his idea any further.
She agreed to head to the Citadel to help his resolve this matter. She did have a plan herself though. It was risky but she had to do it. She was planning on talking Garrus out of killing Sidonis.
Because she taught him. And he needed to learn what she had in mind.