Observe
Shepard was trying her best at hiding the fact from the rest of her crew that she'd fallen for Garrus. He had said to try to not disrupt the crew, but there were so many ears on the Normandy. Somebody was bound to pick up on something, even if it wasn't very significant.
Unfortunately, there was one person on the Normandy who knew everything about everyone. She was the number one source for gossip traveling all over the Normandy. That person was none other than the silent Kasumi Goto, who wasn't all that silent when it came to gossip. Shepard – who was doing her usual rounds, checking in on everybody – discovered that the clever Miss Goto had known about Shepard's feelings for some time.
"The word is that Garrus really likes you". She stated this the moment Shepard walked in to Kasumi's room.
"I can never tell with turians. I think you should go for it. A lot of people want to see you two together". In trying to calm Shepard down before she freaked, she offered her support.
Shepard was grateful but told Kasumi not to tell anyone. Shepard used a serious tone in her voice so that her message came across clear. She was certain that if this got out, the complications would be unbearable.