Lock Notes in OS X 10.11.4
With the OS X 10.11.4 update you can now lock notes on your iPhone, iPad and Mac. This uses Touch ID on your iOS devices and a master password on your Mac. Oddly this is not the same password as your Apple ID, so you have one more to remember.
The video below shows this process on the iPhone, and you can also lock and unlock these on your Mac.
You can use this guide to learn about how you can lock and unlock notes on Mac with OS X 10.11.4.
You don't need to lock notes, and you can lock individual notes, while leaving others completely unlocked.
If you share a computer with your family or with a room mate, you can use this for added security over your notes.