It requires a lot of CPU so there isn't anything silent about it. If some bad advertisement absolutely kills the tab you're looking at, I hope you'd notice. If you're using a multi-process browser (like Chrome) the impact is pretty minimal.

That's not to say it's not bad -- the exploit works remarkably well -- but it's not fast or easy.

Imagine: You open the browser, leave it open on some page and then leave your computer for a few hours or overnight. You can’t notice what a rogue ad did while you were away. Your passwords are maybe gone.

Besides the "hiding" strategy (bots hide themselves under the windows clock. It's a wonderful world out there), I'm afraid it's unlikely you'll notice. Reliability of modern consumer software ain't all that great.