I'm not sure how the device works exactly, but many, if not all, cars these days do have a built-in feature that tells you to put your seatbelt on when the car is on with a little beep or ding noise if you start driving and you or a passenger don't have their seatbelt on
It can also happen if the car is on and idling, but not moving
I would guess that the sound of the device and the seatbelt warning are the same or similar enough
It looks like the device could be motion-activated? But I feel like unless this is explained by the author, it's a pretty confusing plot hole
It could be Heeseo purposefully making it something that Taekyung would notice, but I dunno
Ok. Im a little confused. He heard a beep sound, thought it was for the seatbelt? But actually it was the listening device turning on when he turned on the engine? Or does the device turn on when he sits in the car? So back then when he was drunk and didn't even turn on the car but he heard the beep and thought of putting on the seatbelt.... am I making sense?