You'll never find any traces because none are left
These browser-based mining scripts are written in JavaScript, which means they are just running out of the box in your browser, without any need to install anything. It also means they are only running as long as you are on the website in question.
How about google chrome running in the background after closing the browser? they could keep mining as long as your computer is on, I need a plugin to detect these mining sites and then stops them. this explains the hash rate of coins changing every second, people come and go. imagine if 500 million people could use 50% of their CPU knowingly, could the hash power grow even bigger than Bitcoin's? mild lol.
Google Chrome running in the background after closing the browser is
not website scripts running in the background. You'd probably have to ask Chrome's dev team for what is actually happening, but my educated guess would be mostly garbage collection and update processes. Either way, a website can't simply run JavaScript in the background once it is closed, that's simply not how this technology works. If you want to stay safe from browser-based mining just run JavaScript blocking extensions such as ScriptSafe and you'll be fine.
Hashrate fluctuations are more likely due to miners pointing their hashrate to whatever coin is currently most profitable to mine, this is not a new phenomenon. According to the link these sites made USD 43,000,- within 3 weeks, which equals about 500 XMR @ USD 86,- which makes XMR 23.81 a day. With the current XMR network difficulty and block reward that requires about 1.3MHs which is about 0.5% of XMR's hashrate. Of course XMR is only an example here and some variables may have slightly changed since AdGuard collected their statistics, but that should give an idea about how much (or how little) hashrate we're actually talking here.
There are more and more sites that use up a lot of my CPU.
Yeah, you can thank bad coding practices and questionable design choices for that...
