If the key changed, aka a new different device is visible, you know mail has been dropped in, very clever !
https://hackaday.com/2022/05/30/check-your-mailbox-using-the...
I wonder if the creator had neighbourhood style mailboxes down the road? If not this seems quite complicated solution for an object that is probably with range even BLE.
I tried building a mail sensor a couple of years ago where the mailbox was a fair distance from where I was living. I was not able to create a solution that didn't either have false positives or false negatives. For an outdoor object jostled by wind and rain it is harder than it seems.
Something that would let you send extremely tiny (<1kB) packets, using a wireless protocol that could be implemented extremely cheaply, piggybacking on the bandwidth of nearby internet-connected devices in a privacy-preserving way.
Amazon has a network like this called Sidewalk, using Alexa devices as gateways, but I don't think it's very open to third-party experimentation, and it's definitely not an interoperable standard on the gateway side.