The presence of the hair in the trap. The likelihood that a hair would have shed, plus found its way in the drain, plus got caught and not washed down is slim to none in my opinion.
Someone elses hair was shed and caught and not washed down, why is it hard to believe hers wasn't?
Read the whole scenario. She was playing there. Apparently we lose one hair every fifteen minutes. Assuming she was there for only a short period of time she would only shed one hair. This one hair had to make it to the floor and not the carpet. It further had to be washed up by Westerfield and put in the drain and it had to avoid getting trapped (not all hair gets trapped). The sum total of all of these probabilitiers is almost 0. Adding more time spent increased the number of hairs normally falling but it would still multiply out to nonexistant.
Add to the hair, the other improbable occurances and its a very remote possibility the evidence came from a short visit while she played.