This is one of those things that may or may not bother you, but ferrets also can get into the darnedest places when allowed to roam inside. My cousin's ferrets would do things like climb through the drawers/shelves in the kitchen. I'm not entirely clear on how they found their way inside the counters, but I remember that they had to wash all of their silverware after the ferrets got into the silverware drawer. I suspect it's very difficult to keep them out of specific parts of a space because they can get through such tiny openings and like to explore.

Depending on the construction of your apartment, it might even be possible that they'd escape into a neighboring unit through some small opening. I don't know how you could easily tell if that would be possible without actually letting ferrets loose and seeing if they find one, but I could see them getting into the walls somehow through some small opening at the back of a cabinet or something and leading you on a very stressful adventure.

And yeah, they really, really do smell if they have their glands. Many years ago, I used to work with gland-intact ferrets in the petting zoo/outreach portion of the local zoo as a teen volunteer, and they definitely had an odor to them. One of them also was a biter despite daily handling. I was not a fan at the time. (It was also a case of my not wanting to work in the petting zoo, but rather to work with the behavior team in creating/designing/building things for animal enrichment, so the whole thing was pretty much a fiasco of a summer, but I remember particularly not liking working with the ferrets.)