It wasn't the unflushed toilet alone. It started with the door being open and no one being home. Strange, but not overly concerning as the door latch was broken. She calls out and nobody was home. Maybe someone forgot to lock the door. Then the blood in the bathroom. Another curious finding, but it wasn't a bloodbath so, just strange. Then comes the unflushed toilet. She sees it and thinks Laura and Filomena wouldn't leave the toilet unflushed. What if someone came in while she was in the shower?? Then she feels a "lurch of panic". It was the totality of all these findings that caused her to be concerned. The poo was just the final straw.
Everything had been pretty cleaned up outside of Meredith's room and Filomena's room, whether that was by "natural" reason or by someone cleaning it. The other things you mentioned, could have been explained by Meredith being home or Filomena being home. Unlocked door (one of them forgot to lock it), blood (she already said "menstrual," that could have been Meredith or Filomena, poo (their guy-friends). Don't you find it odd that she just "calls out" and thinks "nobody was home." Why wouldn't she go and really check the two closed doors? At least knock on Filomen'a room, if no reply, open it. It wasn't locked, or do we know have to have another locked door? Then she would have obviously seen the open window. Knock loudly and yell out Meredith's name repeatedly by her door. Does she say she even checked Meredith's door, or does she just say she "called out" from the living room?
I just can't go from seeing the poo, to not seeing the broken window (then she would have known for sure something was wrong.) Because she would have gone and checked Filomena's room to see if she was there. And checked Meredith's room, but only very conveniently it happened to be locked so she couldn't tell for sure.
There is something very wrong with her story. It is causing alarm bells to go off all over in my brainz.