I had a lot of trouble getting this one to pop up and couldn’t figure out why. In the end I had to do a dedicated run for because it was the only challenge I was missing in Bangkok.
Well, anyway, I got to Dexy without being spotted in the way, injected her with an emetic poison, subdued her and dumped her body in a dumpster. Challenge popped up as completed and I thought that was it, so I went ahead running aimlessly and got spotted by a camera. I exited to menu and behold, the challenge wasn’t completed there anymore. So I load up a previous save, drug > subdue > dump again, no second challenge pop-up or anything but I exited to menu right away and there it was now.
I don’t know wheter it’s intentional or not, but it seems it’s not only the part where you get rid of her where you have to avoid being spotted but the entire run before and after that. Putting this here just in case it would help anyone