It's an old story, web browser meets javascript, they head to a mine with your CPU in tow and they all decide they like it so much they pop under again and again without telling anyone.  There are many websites out there that use your CPU for alt-coin mining while you browse their site but a few have gone back to the old trick of the hidden pop under window to ensure you keep mining for them after you think you have closed their site.  The Register links to some more information on this practice, though not any of the guilty parties.  It is not hard to resolve this issue, just make sure you check task manager after closing a browser and terminate any remaining processes; a visual clue it is happening is that the browser still shows as active on your taskbar even though no windows will show when you click. 

"Miscreants have found a way to continue running cryptocurrency-crafting JavaScript on Windows PCs even after netizens browse away from the webpage hosting the code."

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