If your a fan of Aircraft Tracking, I highly recommend the article from Global Investigative Journalism Network (or GIJN) “Planespotting: A Guide to Tracking Aircraft Around the World“. This is one of the most comprehensive article I have seen on the subject, addressing it from multiple angles (#avgeek, journalist, activist, …). They even propose a short version if you just want to start tracking.
An interesting article about how Buzzfeed is training an Artificial Intelligence (AI) to detect planes circling over San Francisco (mainly law enforcement and other contractors).
That’s an interesting direction… I will have a look at how to do that and detect anomalies in trajectories on my trackers. I expect to see more planes circling due to delay in the local airports (Hong Kong, Shenzhen, Guangzhou) than spy planes!