When I close the lid of my MacBook Air, it's almost always plugged in. When I open the lid, I have to sign in and my screen is clear of all the things I wanted to see or was looking at, but did not finish. This happens several times a day. I have to close the lid because of a possible dust situation where I need to keep the computer during the day. This just started happening a couple of weeks ago. I am running OS X El Capitan. I do not log out, but it thinks I did. Grrrrrrr!


Sounds to me like the cause of it is this setting, which you'll want to disable:

System Preferences > Security & Privacy > 
Require a password X minutes after sleep or screen saver begins

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