Why is it in Hollywood movies every time there is a vent or AC unit someone has fixed little ribbons on it? Is that a thing americans do? Why? The vent in this movie is close to the floor in the toilets of a train. This implies someone got on all fours ON THE FLOOR OF A TRAIN’S BATHROOM to stick those ribbons there for a reason i can’t even begin to comprehend.
The answer in movies is the same as IRL: it’s to visually indicate that air is moving through the vent. In a movie this can be hard to tell the audience without a possibly annoying sound. In real life you might see this on vents that are difficult to access, probably so an HVAC technician can tell if the vent is working.