Yeah, there are hundreds of different ways to do this without people knowing. I thought this one was interesting since there was a college that had a project like this a few years ago that used Raspberry Pi's to do the same thing but it was "found out" when a student found what they thought to be a rogue/abandoned raspberry pi plugged in somewhere in a library or something on campus then set out to reverse engineer what it was being used for. Im fairly sure that was on Reddit but I remember being linked to it from another forum.