That's an excellent and practical question for urban management. For a high-theft area trash can, the "most secure" mechanism balances maximum deterrence with practical public access and maintenance. Based on security engineering principles, here are the top contenders:
The strongest single point of failure is often the latch point. A heavy-duty, shrouded or recessed hardened steel padlock with a short, thick shackle is a classic choice. It resists bolt cutters and picking. However, padlocks can be vulnerable to leverage attacks.
For a more integrated solution, consider a heavy-duty slam lock or cam lock built directly into the bin's design. These are often made of stainless steel and have minimal external protrusions, making them hard to pry or grip. Look for models with anti-drill, anti-pick features. The key here is that the locking bolt engages deeply into a reinforced strike plate within the bin's body.
In some modern deployments, smart locks with audit trails are emerging. These require a unique code or digital key for maintenance crews, providing a record of access. While high-tech, their electronic components must be exceptionally weather and vandal-proof.
Ultimately, the highest security often comes from layering. Combine a robust internal slam lock for the main compartment with a secondary locking system for service hatches. The bin itself must be anchored to the ground with a high-security concrete anchor to prevent the entire unit from being carted away.
Therefore, the most secure mechanism is typically a vandal-resistant, integrated slam lock system on a permanently anchored, reinforced steel bin. It removes easy external points of attack while maintaining the necessary functionality for city services.