Fred thought about it for a moment, then shrugged and nodded for Roger to open the door. Because honestly, a situation like this, creepy house with dust everywhere and someone - obviously a wizard since the house was on the floo network - walking around when they shouldn't be because everything else looked deserted? You had to investigate. Or, if you were a Weasley and an Order member, you had to investigate.
It could have been a muggleborn on the run, or something.
Roger opened the door, and when Fred looked in he was surprised, though, to see an old woman. Old as in ancient, shuffling and turning to look at them from the dark sitting room.