(Disclaimer: I work on an app that tries to do that. Ideally privacy preserving. And we implemented it using Google Nearby API to detect whether people are clos

(Disclaimer: I work on an app that tries to do that. Ideally privacy preserving. And we implemented it using Google Nearby API to detect whether people are close. But that sends all messages over Google servers and that would be a deal breaker for us, but the tech itself would be really cool to have. Let me know if you can help.)