Hi -midnight,
The spying is going on behind your back and what is going on is hidden in Eula's. Code is being injected with a fixed ID part to know from where you came from and where you are heading on the Interwebs all of the time, even on sites that are encrypted. The Internet is free space only because we, the users, are the product and all we do there is being sold and will land on some marketeer's desk to be resold again to another pimple face to earn some more cheap money on your browsing habits and history.
Not all extensions do this off course but it is known the hoverzoom add-on for instance is such an extension. Sometimes shady characters try to take over extensions from developers to start injecting these devious tracking codes. Google allows all this going on for as long as it is “clearly disclosed”. They already lost some ground in firefox, but I fear other parties are also deep into this tracking and fingerprinting.
Only through analyzing your Internet traffic you can see what is going on under the hood and with a monitoring tool like fiddler we can find them out.
polonus