There's an app in my phone that shows local train timings and stuff. One thing that fucks my mind up is, even if my WiFi is turned off, it can still detect where I'm at and if I'm at train station, it pops up a notification that this station has free/public WiFi and I can use it.
HOW THE FUCK YOU FOUND OUT ABOUT FREE WI-FI IF MY PHONE'S WI-FI IS TURNED OFF.

Maybe the app just knows which train stations have free WiFi? (As in it has that info in some database, and thus does not need to scan for WiFi). And if your position, which is tracked by GPS ir something, is at one of the train stations, it provides the information about the free WiFi?

Well sometime Facebook showed me like 8 open Wi-Fi's on a map on my way from work to home. And I don't mean just like cafe's wifis, but like personal connections as well. Didn't have wifi or gps turned on.

@Aitkotw that things messes me up too. But I guess they can scan for networks of when you're WiFi is turned off

Android lets to scan for wifi signatures around you even when your wifi is turned off..

I actually played around a little with sim cards over the christmas/new year breake.
MQTT/USSD is quite nice.
Now to what you're asking:
Option 1) your phone lies and the wifi isn't off, not really. Something that iOS does for example
Option 2) the app knows where free wifi hotspots are and shows you a notification once you're near them. You may ask how does it know? Besides the gps location, it could also use the nearby cell towers to track you! How you ask? https://developers.google.com/maps/... it's quite scarry how accurate readings you get...