"A company can often detect or control when an outsider (non-employee) tries to access company data either physically or electronically, and can mitigate the threat of an outsider stealing company property. However, the thief who is harder to detect and who could cause the most damage is the insider--the employee with legitimate access. That insider may steal solely for personal gain, or that insider may be a 'spy'--someone who is stealing company information or products in order to benefit another organization or country. [...] This brochure serves as an introduction for managers and security personnel on how to detect an insider threat and provides tips on how to safeguard your company's trade secrets."
Office of the National Counterintelligence Executive: http://www.ncix.gov