This is a very basic question, a trooper stands on a rooftop where various enemies have LOF against him from below. If the active played activates said trooper (one on the rooftoo) declaring the short movement skill Move and going prone, would the troopers on the ground have LOF against him at any point from declaration to movement trajectory?
Much like stepping behind a corner, enemy models can ARO against the active model from its initial position or anywhere along its movement. So yes, they could still ARO from your position at the start of the order, before going prone took you out of LoF.
Model A is in Point A on a rooftop. Enemies A and B can see him there. Enemy C doesn't. Model A go Prone and move to point B. Enemy C would see him if Model A wasn't Prone. Can Enemy C declare ARO? Is that? If so, no he can't. Remember that you can go/cancel Prone only AT THE START of a Move Short Movement Skill.