The flip side is also hard to quantify. There is no way to show that if Officer A was sitting in this particular parking lot, it prohibited a vehicle from being broken into or stolen and the would be suspect went to another location and did what they were going to do.

Does marked units patrolling reduce crime? I don't know. It is a basic tenet of law enforcement that visible, marked patrol vehicles with uniformed officers deters crime, but there isn't any way to prove it.