Isfet's gloriously bright red hooded robe hid most of his body, and all of his mane, too -- all she could tell was that his horn and chin were white. Not exactly defining characteristics, and 'Isfet' surely wasn't his real name, either. If Isabelle went and told the ponice that the leader of a major magical criminal organization in Canterlot was a white unicorn colt, they'd just shrug their shoulders and send her on her way. Probably laughing in her face as they did it.