WiFi was turned off completely on the phone, also message itself was a native UI dialog, not something on a webpage.
25 karma · joined January 15, 2016
I wish I've taken a screenshot of it!
At the time I just dismissed this as some tinfoil hat developer adding some nonsensical warnings to the firmware, but in retrospect, after reading this article - this matches perfectly, chances are - phone was indeed detecting Stingrays. Still no idea how it managed to do it.
EDIT: I had no data/IP connection of any kind at and around the time of seeing this, so this is clearly unrelated to TLS interception.