by Turncoatwhen he was happy, he'd leave his house (so go figure they never saw that side of him).
Most people, who see a psychologist or psychiatrist, usually attend therapy on a regular basis over a period of time. So, I'm not sure how a psychologist could have missed that piece of vital information.
I personally believe that a thorough investigation into the patient's behavior should be conducted before any label is slapped on them. For example: talking to the person's parents or teachers if they're young. What makes that difficult though is the anonymity agreement between patient and psychologist.
I suppose it's not that hard to miss things if all you have to go on is whatever the patient is telling you, but that's where discernment comes in and specific questioning that may reveal other possible causes.