A bug with the Apple’s iPhone 6S and 6S Plus could leave your information vulerable. The issue it with the 3D touch feature and Siri.
If you have Siri activated on the lock screen, someone who has your phone in hand could use that function to search for something within an app, then push down hard on the screen and end up getting access to functions that would typically require a password. This affects devices running iOS 9.2.1 & 9.3.1.
Until Apple comes out with a fix, your best bet would be to disable Siri on the lock screen. If you really enjoy the feature of Siri on your lock screen, make sure you don’t let your phone out of your sight.