Some people experience depression much different than the average person.  Some get real busy and experience a combo of depression and high anxiety.  This could eventually lead to burnout at work and sometimes all areas of  life as they become exhausted with it all.  They will not look depressed, but one clue is how busy they are and how they begin to withdraw from friends and family; to isolate themselves except when it comes to work and other busy projects.  Professionals should be careful not to miss possible bipolar symptoms.