As to being "unnaturally" calm and self-controlled: I suspect that authority figures find this threatening, especially in emotionally-laden situations, as the person who is not overwhelmed by emotion is the one in charge of the situation. (Except, of course, when he/she is not really.) I hesitate to discourage adolescents from managing their emotions in time of crisis, but I do think it may help to recognize that this is experienced by some authority figures as undermining their authority. Therefore, it may be more effective when accompanied by overt gestures of respect and deference.
Not sure I agree with the threat perception. My take is that law enforcement are specifically trained to look for "suspicious" behavior, which can basically be anything that isn't expected and normal. They have a generalized notion of how a child of a certain age
should behave in a certain situation, based on training and personal experience. Because our children are statistical outliers by nature, their behavior will not match. Ergo, suspicious.
