UV-C sterilization lamp
Staying at an Airbnb home means potentially exposing yourself to the homeowner in ways you never imagined. You need to be mindful of hidden cameras in bedrooms and bathrooms. Also, if you notice any faint lights in the vent like the OP did, it might be time to pack up and rent a hotel room instead.
Actually, this particular case shouldn’t have been a cause for worry (maybe). The light was being emitted from a UV-C sterilization lamp, which is used to deconstruct viruses and bacteria at the molecular level. Does it work? Industrial-grade UV-C lamps do, for sure, but residential ones that fit in vents? That’s up for debate.