For a professional force like Russia, you don't expect stuff that won't happen and goes with it. For starter, How do Russia sure there are no Ukrainian asset in the Area? Would you care to explain to me? I run intelligence with the US Army for 3 years, I never be so certain that there are no enemy asset in any area I operate, remember Robert Ridges? AC-130 with IR also called ahead and say there were no Taliban, do you know what happened in the end? So, how can you be sure at any point there are no enemy that can touch it? You do know camouflaging a position is a thing, right?
As I said, the area is NOT secured, Russia does not have Air Superiority nor Ground control, which mean doing stuff like this is taking a risk that not going to fly, yet they did it anyway, you don't "Assume" you're not going to get hit if you are a competent force, you don't assume there are no enemy asset in the area as situation is dynamic, and as a Army Intelligence Officer, I can tell you, unless Russia have some sort of "God Eye" system, you can't sweep the area with drone and say "Hey there are nothing there"