I log in my etherwallet and leave it, but it automatically logged out. I've already logged in my etherwallet many times and even I just close the window, it is logged out already, maybe you leave your account without closing or shutting down your pc while your friend is beside you and got a chance to steal what is in your wallet. Maybe you leave your pc with your friend for a couple of minutes then he got a chance to move your wallet into new. Or maybe your friend has a copy of your private keys. Think of the possible way why that happens.