Plot hole: During the beginning moments when the couple go to the top floor, as they are going to the roof they discover a lot of smoke on the penthouse floor which leads them to seeing the charred corpse in the hallway. The only problem is that with the smouldering body wouldn't it have set off the smoke alarm or fire alarm? The building is a very posh apartment block and they are on the penthouse floor making the likelihood something as basic as a fire alarm would be triggered. The killing was more in the heat of the moment and most likely the killer burned the body then and there making the probability of a lot of fire on the floor very high. Even if the woman had been burnt somewhere else and brought back to the floor there is no way the smoke around wouldn't at least have set off the fire alarm or sprinklers.