I have post-nasal drip & chronic rhinitis. What this means is I cough all year round. 90% of the time I'm not infectious, it's allergic catarrah irritating my throat.
For the 10% of the time that I have got a bug, 90% of this time I am aware of this & won't be out at a restaurant.
So if you heard me coughing behind you, there is only a 1 in a hundred chance you would catch anything off me. & there are lots like me, allergic rhinitis
& PND are common,
So I agree the woman in question should have shielded her cough, but I think she may have been aware that the fact of her coughing was the reason for your hasty departure, & that's a bit judgemental when there's a fair chance she is someone who just coughs a lot, & knows she's not going to spread any bugs at that point.