Acid In The Gut.

The soul prowler's method of perception has all the problems of the spiritual echo method including the short range and many other disadvantages. The soul prowler essentially has a sight disability. If not for the fact that it felt Legion-7's probing with the divine echo, then it wouldn't even know that someone was approaching it. Even then, it couldn't sense Legion-7 because he was too far away from it much less tell his position. 

His divine echo perception technique outclasses this shoddy perception technique in every way. But Legion-7 still complained. "The divine echo method is still lacking. Even the lowest soul prowler will be able to sense me when I sense them."