Where's the story? You spend a few paragraphs describing this thing but…then what? It's just a thing that kills you, and that's not going to get you very far these days.
The addendum is useless as it just emphasizes that the thing that kills you will uh, try to kill you. That's not terribly new or interesting information.
Agents were alerted by a report of a possible murder in which a family of five (5) went missing over the course of eight (8) months.
So, a family spent almost a year with this in their house and nobody within the house figured out that it was eating them one-by-one over the course of that time? Were new missing person notices being churned out every two months or so? "Welp, lost another one!" How do we know that the chair took 8 months to methodically and slowly kill every member of the family, again, without anyone noticing?
Once the subject is sedated, SCP-XXXX's 'backrest' folds forward onto the victim, encasing them within it's membranes. Once the subject is encased, SCP-XXXX will begin to secrete enzymes to digest the subject. Digestion has lasted from ██ hours to █ days.
Why did the family act like nonchalant Sims when this horrible thing was happening to them? "Where did your father go, he was just right there but when I turned around twelve hours later, he was gone!"
The only other way to read that is that they all sat down on the chair at one time, got snacked on simultaneously, but nobody noticed that the family was gone until 8 months later, which only raises further questions.