Coughing on a night is generally a symptom of asthma not Covid-19. As you say she stops coughing when she's outside I'd assume asthma/allergies rather than Covid-19. There are reasons other than Covid-19 for coughing. As an asthma/allergy sufferer I'm starting to get a bit tired of people side-eyeing me every time I sniff or reach for my inhaler. I do not have Covid-19. Half of the time I am allergic to you and your cat hair covered jumper and overpowering perfume.
She coughs all day, continually and occasionally wakes me up coughing at night.
When I say that the coughing only stops when she goes out I mean that I can no longer hear her coughing through the walls because she is no longer there for me to hear.
Apart from exercise and essential shopping trips I am staying indoors. I do not follow my neighbour around to monitor her cough. On the 3 occasions when I have seen her outside she has been coughing but at least she is wearing a mask.
Last night I couldn't hear her coughing and was hoping she was feeling better / sleeping. She was out and about somewhere and came home about midnight. I could hear her using the lift and her front door opening. The walls are thin and I could not fail to hear her if I wanted to.
Of course I do not know whether she has the virus, just that she has a new, continual cough. Also her voice has changed over the last few days, it has become much deeper, croaky and kind of gravelly.
I also suffer terribly with allergies, my worst allergy (I was tsted in an ENT hospital) is to silver birch pollen. There is a silver birch tree outside my window and it oppresses me daily. My eyes itch and water all day long and I occasionally cough, the cough is worst at dawn and dusk.
These are exactly the same symptoms I had last year, so I do know about asthma and am familiar with feeling embarassed because of hay fever symptoms during the current crisis.
My syptoms are nothing like my neighbour's.
I have no idea whether she has the virus or some unrelated new cough however I believe it is entirely reasonable to be concerned about her behaviour under the circumstances