The footage at the beginning of the episode where Flack is shown getting out of a squad car is from 3x24 "Snow Day" and his shirt is a completely different color when he shoes up in the hallway and finds the blood door.
(at around 50 mins) Mac concludes that a hair could have mutated and therefore it would not have matched the suspect DNA on the scene. While theoretically possible, it is unlikely that a hair bud had undergone such mutations that its DNA would not be sufficiently similar to its owner's DNA. Such inconclusive result would require a more thorough comparison.