[np1810]

Cool idea and this was nostalgic as well... I created a similar application to communicate and send beacon signals over SMS (using Twilio) without Internet (as internet went down) during emergency SoS situation motivated by 2015 Chennai floods...

Suggestion: You can possibly try if you can use OpenHaystack [1] to send some "unknown tracker found nearby" alert to any nearby iPhone even if the receiver device doesn't have your app installed or use the FindMy network to send arbitrary data for your app's communication. [2]

[1] https://github.com/seemoo-lab/openhaystack

[2] https://github.com/positive-security/send-my