Here are two ways you could be tracked after attending a protest

The first story comes from Hong Kong where South China Morning Post reports that, according to sources, "police have used court orders to obtain digital fare payment details and CCTV camera footage of anti-government protesters from the city's largest bus company." As ‎Maciej Cegłowski (who has been posting from Hong Kong for weeks) points out:

Forget combing through records; police have had many opportunities this summer to arrest 500-1000 people in person. But they let them go. The real power comes from retrospectively being able to dangle a potential 5 year sentence over anyone they decide attended an illegal protest

Of course, that process contemplates the police using the courts to get information. Buzzfeed explains you can track someone just using public webcams and Instagram:

authorities could use publicly available webcams filming an area where a protest is taking place. By looking at Instagram stories that protesters post in real time at any particular location, authorities would be able to identify them and track their movements throughout a city using other cameras, including before they arrived at the protest, and what they did afterwards. Other people featured in the background of the Instagram posts could also be identified — by facial recognition technology, for example — and followed in this way.

In our experiment identifying people using their Instagram stories, we started with camera footage from tourism and surveillance company EarthCam, which streams live footage online for free and also makes previous days' footage available for anyone to access.

Buzzfeed compared streaming footage from EarthCam to Instagram stories posted at the same time. They were able to track a test subject's movements through Times Square in great detail:

Once we identified Hayes in the video footage, we could map exactly where he moved throughout Times Square and observe what he did, even if he didn't share it online. For example, Hayes took a selfie with the Naked Cowboy, but he never posted it. But we know he took the photo from monitoring EarthCam's footage.

You can check out the the high resolution footage highlighted by Buzzfeed here, and see several more livefeeds at Earthcam's site.