Firstly, you need to determine the level of severity. Was any real damage done? Also consider his attitude. If he is humble and wants to correct the issue, work through it. If he has a bad attitude you may need to raise the level of pain with regard to the action you take. Let him know up front that accepting responsibility for his actions is a key leadership trait that is important and speaks to his character.
Consider using a counseling statement that looks something like this:
___{Insert Rank/Name}, on or about ___{Insert Date/Time} you committed a physical security violation in that you {Insert specifics of situation. For Example: you failed to lock up the classified safe prior to your departure for lunch}. Your lack of attention to detail placed classified information in a vulnerable position. As a result of your actions 3 unit members had to conduct an inventory and page count of all classified materials. We lost 32 man-hours as a result of your carelessness. We also had to submit an incident report to higher headquarters. You {Select statement that applies: accepted responsibility for your actions and worked to overcome this mistake or have consistently failed to accept responsibility for your actions and insisted on blaming others for your lack of attention to detail. Furthermore you have displayed a poor attitude with regard to this issue}. As a result of this situation and your behavior I am directing the following corrective actions (Select all that apply);
