Also, how do laws treat waves in public (radio, light) - do they get the same "privacy" treatment as people (that is, no expectation of if in public)?
Edit: actually since joining/browsing an open WIFI network can be considered criminal...
I believe this area is legally fuzzy but courts have determined that police cannot drive around with say, a thermal imager and observe en-mass people in their homes. One could argue that it's just thermal radiation you are viewing from public space.
It seems to me that if you really want people to slow down near the elementary school (as opposed to just issuing tickets and collecting fines), this is EXACTLY what you want to have happen. If you put a cop there, and people warn others of it so they all slow down... mission accomplished!
By analogy, if you've got a target very valuable to the enemy (maybe a stadium for a high-profile sporting event), having phones alert users that they're being monitored might help to ensure that nobody tries anything during the event.
To be really effective, it would be seem specific and personal: text messages that say things like "We can see you. We know what you're up to." Stuff like that.
Do you think it might get triggered on your phone by the presence of the anti-porn filter that UK mobile provider have? For instance, when trying to access "adult" website (I use quotes as sometimes, even non porno website gets filtered out) you end up on your mobile provider page blocking you from the actual website your looking for.
I wish I've taken a screenshot of it!
No exotic measures are required to do this.
I installed my own certificate on Android and it warns me about it each time after it boots. It's quite annoying, because there doesn't seem to be a way to disable it (without modifying /system) and I trust my own certificate much more than any other the phone comes with.
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.