The iPhone may seem completely secure when locked, but there are still some items that can be viewed. This includes Notification Center, which may be showing private information, access to Siri and the ability to send messages.
To limit what can be accessed, open Settings and select Touch ID & Passcode. Enter your passcode and scroll down to the Allow Access When Locked section. From here you can turn off any option that you don’t want accessible when your device is locked.