The security desk is located behind reception on the 2nd floor. Although they may not be present at that location while patrolling or providing first aid etc., they will post a phone number where they can be reached.
The easiest way to contact them is by phone. Their number is 604-844-3838 or 3838 from any local phone. They can also be contacted by picking up any of the security phones located at various locations throughout the campus: