you have to search around the area for the flask so I really don't understand what you're talking about. Each of those detective things gives you an area and you have to find something that you can use to track the person within that area and then scan it in. It is not as simple as pressing the night vision button and then Batman finds it on his own as you make it sound.
also, how large of groups have you fought, because it isn't that easy when you have a group of thirty thugs, some with knives, cattle prods, and a gun locker they can open up to get assault rifles.
...









