Drew only felt a prickle of uneasiness which began between his shoulder blades and ran a chill down his spine, as if rifle sights were on him.
Drew realized that he had walked in on something though he wasn't quite sure what. 
mnli hypothesis: Drew realized that he had walked in on something though he wasn't quite sure what.  premise: Drew only felt a prickle of uneasiness which began between his shoulder blades and ran a chill down his spine, as if rifle sights were on him.