Each class, he paid careful attention to the students" affinity as he walked among them during his lecture. He mentally noted that Lovegood had gone from a perfectly neutral Grey (so rare among wizards and witches, to have such a perfect balance of Light and Dark magic) to slightly Dark-Grey. Not much, just a little shift. The eldest Creevey boy was edging more toward Light Grey (muggleborn and muggle-raised magicals, he noticed, usually were more Grey). Smith, a 6th year Hufflepuff, was once more leaning to the Grey, having shifted slightly to Light the year prior. Each student's affinity was carefully stored in his memories so the "Dungeon Bat" would notice any sudden shifts. (A sudden shift in affinity, especially a shift towards the Darker side of the spectrum, usually meant the witch or wizard suffered a traumatic experience. He may be seen as an evil git, but Severus hated seeing a student suffer from anything that would cause a sudden shift.)