Do athletes ever mask a condition or not tell their doctor about an issue?

Sure. Part of the debate about whether or not the history and physical exam is adequate enough is about whether the athlete is being truthful and honest. We base a lot of what we do and how we proceed in clearing athletes based on what they say is going on. So if they're not being truthful in their responses, or maybe they think it is normal for an athlete to get short of breath, or to get mild chest discomfort with their athletics they might not say anything. If we don't know about an issue, it can lead to difficulties in diagnosing underlying cardiac diseases.