Earlier quoted context omitted.
My dog recognizes different cars by their noise profile. So he'll wake up from a nap and get excited whenever my wife pulls onto our street, but stay down for any other car. It probably helps that we're the only prius on the block, but still impressive!
I heard about a test someone did, after wondering how their dog was always standing in the window at the end of each workday, anticipating their owners arrival home. After casting about, a theory was developed that the dog "smelled time". That is, as the day wore on, the scent of their owner, decreased in the air; at a low-enough concentration, the dog knew it was about time for the front door to open and their owner…
Maybe a sealed box that opens to vent the scent. You'd need to have the same box do the same, with no scent too.