From the DOT Publication Best Practices for Random Drug and Alcohol Testing.

Every employee should be discreetly notified according to your company’s policy, but random testing must also be conducted in strict confidence with a limited number of people having knowledge of the selection list. Why? Because it helps maintain the element of surprise. Best Practices: Every employer should have procedures in place to ensure that each employee receives no advanced notice of selection. But, be sure to allow sufficient time for supervisors to schedule for the administration of the test and to ensure that collection sites are available for testing. Remember: Employers must provide appropriate privacy for each employee the fact that he or she is being tested.