As a physician, it was the Medic’s job to observe people to see if they were showing symptoms of any illnesses they were not willing to discuss with him in person, so it didn’t take long for him to notice certain things.
The Sniper, for example, never seemed to present his genuine self to anyone. His interaction with the team members were based on their initial impression of him, and he in turn remained at the barest level of acquaintance.
It wasn’t a surprise that a hired gunman would keep an arm’s length from everyone. With the fluid nature of the battles and the high turnover rate between both teams, many often found themselves being transferred to the other side of the battlefield. Everyone took to being forced to shoot at their former teammates in a different manner, and the Sniper in particular seemed determined to avoid forming attachments.
The Medic didn’t care about the Sniper’s various facades. If he wanted to pretend that he had no feelings for anyone, and alienate himself from the rest of the team, great. That made it all the easier for the Medic to work his way through the Sniper’s defenses.