3 hours ago, somebody stole most of my coins out of Metamask. For 2000 dollars!! For the second time! In August for the first time. But then I left my Metamask unlocked so I thought this was the reason. Now again, while I always unlock, they stole it again. And you can see perfectly on Etherscan to which address. In my Metamask it's even my 'preferred address". How is this possible?
Someone must somehow have stolen your private key (your secret phrase). You are probably saving your private key somewhere unsafe that someone saw it (on your phone or your laptop where it is obvious)
Most probably the first time you left it unlocked, a person exported your private key. From then he/she has access to your account even when you don't have it unlocked on your laptop. he/she can easily import your private key to his Metamask and open your account freely and transfer to any account whenever he/she wants to.
You must change your private key. The only way to do this is to make another account and transfer your Eth from the old account to the new one to prevent further theft.
Also, make sure your new private key (your phrase) is safe and no one can access or guess its place.