An exemple is the best in this kind of cases :

Let's write this in a command block linked to a clock :
testfor @e[x=5,y=31,z=5,dx=-5,dy=40,dz=-5]
Let's put a comparator to get an output... It will stay off no matter what happens in the zone...

There are a very few cases where you will get an output, but in most cases it won't work... That's weird