The newer devices can locate each other via ultrawideband triangulation too, completely detached from the cell network. They have special chips for this built-in: https://www.pocket-lint.com/phones/news/apple/149336-how-app...

It’ll show up in Find My even if it can’t connect to the internet. BLE and UWB both work offline.

Samsung has their own "offline finding" as they call it, if enabled other Samsung devices/wearables can locate it somehow, brief on how that's done on their site

I wonder how well the relay phone does store and forward, I guess it does otherwise AirTags are useless outside of cellular footprint territory.

I guess sometimes a late ping is better than no ping.