See if you can find out if the kitten has been neutered/spayed yet. If not, that could make a difference, assuming the owners plan on doing so. It will be nearly impossible for you to find out if this kitten pees inappropriately, but that could be another issue. If there is a common hallway that they use to get downstairs, perhaps they are letting the kitten roam? Maybe make friends with them, and see what you can find out?
The only other thing I know to do is get an air purifier for any smells possibly creeping into your apartment and use the cat music - or other noises that are amenable to her - to reduce any mews, or rambunctious play, coming from the kitten that she might be able to hear, since cats' hearing is so much better than ours. At night, something like a heartbeat toy or ticking clock might be something to get her used to, which might just be enough to 'drown out' whatever it is she could be hearing.
It may be that she will eventually adapt to whatever it is she is aware of now. Otherwise, I have absolutely no suggestions. With any luck some other members who have dealt with something similar will come along and tell you how they handle it. However, continue to work on other probable sources anyway - it could be pure coincidence.
The only other thing I know to do is get an air purifier for any smells possibly creeping into your apartment and use the cat music - or other noises that are amenable to her - to reduce any mews, or rambunctious play, coming from the kitten that she might be able to hear, since cats' hearing is so much better than ours. At night, something like a heartbeat toy or ticking clock might be something to get her used to, which might just be enough to 'drown out' whatever it is she could be hearing.
It may be that she will eventually adapt to whatever it is she is aware of now. Otherwise, I have absolutely no suggestions. With any luck some other members who have dealt with something similar will come along and tell you how they handle it. However, continue to work on other probable sources anyway - it could be pure coincidence.