In Apple's zeal to make Siri available all the time, the company set a default behavior you may not expect: Siri's active even when your password-protected phone is locked. Without needing to enter any password, anyone that has access to your iPhone 4S can use the service to text, email, or pull up contact information. Happily, there's an easy fix: go to Settings > General > Passcode Lock, and turn the Siri slider off. Yes, it'll make using Siri a little less convenient, but your data will be that much more secure.