What we know now: Not-Sasha is some kind of monster called the Not-Them, which takes over a person's life, changing pictures or recordings and appearing normal to everyone except one or two people. It cannot affect polaroids or tapes, so anything on those mediums are the old them (Sasha in this case).
In episode 39, the one where the worms attack the institute, Sasha gets separated from Elias and ends up in artifact storage, where she sees the table with the web design on it. She senses someone else in the room, calls out "Hello? I see you!" And screams. The Not-Them kills her and takes over her life, repeating her last words: "I see you".
In episode 40, after the worm attack, where Jon is questioning everyone about everything that happened which he doesn't have on tape yet, Jon mentions that a couple of the tapes went missing. The tapes in question are all of the ones that have Sasha's real voice on them (the one about the doll and calliope where she comes into the room halfway through the statement; her statement about her first meeting with Michael; the one from during the worm attack), which Not-Sasha hid.
Adelard Decker is a guy who worked with Gertrude sometimes (or at least they were in contact), and in this statement (ep 78, about the cousin who gets replaced by the Not-Them), Adelard binds the Not-Them posing as the cousin to the web table. Breekon and Hope are later seen transporting the table out of the house. At the end of episode 35, Breekon and Hope deliver the table and a lighter with a web design to the institute.