When I lived in an apartment I too dried some washing on a small drying rack that sat under the balcony rail height. The only way it could have,been seen is by a neighbour leaning over their railing and having a good look. Others in the complex did the same. It wasn’t obvious and wasn’t flapping in the breeze for all too see. You had to make a bit of en effort to see it.
Am in a townhouse now. We all have clothes lines in our courtyards. I could see my neighbours washing if I stood on my toes and had a look over the fence. To me, a balcony is much like a courtyard. It is your own personal space, accessed only by you. If your neighbours have to lean over or crane their necks to get a look i’d be concerned about my privacy. I dont see the problem if its not hanging from the balcony above, over the rail or strung from wall to wall.