Being Human S00 E13 Becoming Human: Episode 7

"Flatmates by day - undead by night."

The trio becomes certain that Matt was killed in the gym and Christa, transforming into a werewolf, is convinced that her heightened sense of smell will lead her to the body and thence to the murderer. For his own safety Adam has to lock her in a cupboard but next morning after she has changed back they make a discovery.