How did Siri know where this plane was going? The other day something creepy happened to me. My anti-Apple friend has been trying to convince me for some time that Apple are spying on all their users. To prove her point she used my iPhone to ask Siri where a plane in the sky was flying to. And Siri answered that it was going to Delhi. I said that Siri likes to joke around and she was probably just joking. My friend then said if I didn’t believe it, just check the flight schedules. Sure enough, this morning I checked and a flight was scheduled to leave Toronto for Delhi at the time she asked Siri the question. I don’t believe Apple is spying on me, but how did Siri know about that flight?!?!
Actually this is nothing new and isn’t all that creepy (although I suppose that depends on what you consider to be creepy).

Siri has been closely integrated with the WolframAlpha Computational Knowledge Engine for many years now. 

To determine where a plane flying overhead is going, Siri only needs to send a request along the lines of flights seen from current geoIP location to WolframAlpha. WolframAlpha then returns the details to Siri, such as where the flight is going, the flight number, type of plane, altitude, etc. 

For this to work you must already have Settings > Privacy > Location Services switched on - so if you have any concerns about revealing your location then you should check these settings and which apps you're sharing your location with.

If you're interested in learning more about WolframAlpha, you can refer to Wikipedia’s WolframAlpha entry. This provides a great overview, including the fact that other devices (such as some Android devices) also use it.

In summary: Apple isn't spying on you. All that is happening is your iPhone is sending its geolocation to WolframAlpha which in turn is using data provided to it from another third party to return the flight information back to Siri. This is nothing new and some Android devices can also do the same thing.