This is a great property in the SharePoint object model that I discovered today, that indicates the time of the last security related change done to an object in the site collection. This could be a change to any securable object (web, list or list item) or modification to the permission level rights etc. It will work regardless of whether the subwebs and lists use unique permissions or inherited permissions. Adding new objects that don’t inherit permissions also means adding permissions and will result in an update of the flag. The same applies for braking/restoring permission inheritance.
You can use the SPSite.LastSecurityModifiedDate and the SPSite.LastContentModifiedDate properties as indicators of when something has changed for example to invalidate any custom built caching mechanism as we did in out current project and this lead to exceptional performance improvement.
The values of those fields are coming directly from the content database from the Sites.LastSecurityChange and Sites.LastContentChange fields.