I saw someone use this to track his mail state. They have a contact sensor inside their mailbox that rotates the broadcasted key based on the trigger count.
If the key changed, aka a new different device is visible, you know mail has been dropped in, very clever !
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.
I wish we had more / more easily accessible networks that let you do this.
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.
If the key changed, aka a new different device is visible, you know mail has been dropped in, very clever !