Everything points to hallucinations being product of some sound, from suspicious persons wearing headphones at all times, which seem to be more of filters for certain frequency, while allowing normal sounds through; to protagonists having revelations while focusing on one task, thus eliminating sounds from their consciousness, and finally the taxi driver not doing anything but standing around, but still getting caught in the hallucination eventually.
Maybe Yuki would have better results had she not been wasting so much time trying to explain, and just demanded to be driven out of town asap. Or someone would have stopped them on the way, and we at least would have more info to work with.