Something along those lines has been a common idea for a while - it was one of the major use cases for RFID tags when they were still fairly new, for example. I

Something along those lines has been a common idea for a while - it was one of the major use cases for RFID tags when they were still fairly new, for example. I'm surprised that it doesn't already exist, given that I first heard the idea several years ago (possibly >5, I don't really remember).