I work in a building classified as a foreign trade zone. If any door is open for more than a few minutes, I get an alert. All doors automatically lock when closed, and have keycard entry. Any employee that needs to open a door has that permission on their ID badge. If they don't, I can manually open it from my desk, and even give them the permission myself. All exterior doors also have cameras on them.

Extremely effective without being overbearing or intrusive.