Continuity mistake: Barb's eyes are green throughout the movie, but when Chucky stabs her in the eye, the eyeball that rolls down the steps is blue.
Continuity mistake: When Barb first enters the house, Father Frank walks up beside her. The shot cuts. Barb steps onto the spot of blood, and the camera pans up. Father Frank walks up beside her a second time.
Continuity mistake: When Nica finds Ian asleep next to Chucky and shuts off the projector, she puts her right hand against her chin. It then cuts to an over-the-shoulder shot of Ian. When it cuts back to Nica, her left hand is against her chin. (You can tell in the over-the-shoulder shot that she didn't move at all, and thus didn't switch hands.)
Continuity mistake: When the cop calls Tiffany, his mouth goes from closed to open between cuts.
Continuity mistake: When Nica is wheeled out of the courtroom, when it cuts to the wideshot, suddenly she has moved back about 10 feet between cuts and is wheeled through the same area a second time.
Continuity mistake: When Nica stabs Chucky, how deep the knife is in his back changes between shots.
Continuity mistake: When we see Sarah's death, Chucky in the background has the "stitched" face. However, this makes no sense as when this happened, he would have had the regular doll face still.
Continuity mistake: In the flashback, when Charles pulls the knife out, Sarah goes from having her mouth open to having her teeth gritted instantly between cuts.
Continuity mistake: After the wheelchair smashes on the ground, its pieces and the banister pieces change positions next time we see them.
Continuity mistake: When Nica smashes the car window, there's still a lot of glass around the frame. It all vanishes in the next shot.
Continuity mistake: The amount of blood on Nica's clothes changes repeatedly during the final half-hour of the film.
Continuity mistake: When Barb pulls the first few bits of skin off Chucky's face, it's very noticeable when they switch from one Chucky prop-head to the next. Suddenly, the stitching (especially on his lip) has much more detail, and some of the imperfections vanish.
Continuity mistake: When Barb notices the "loose skin" on Chucky's face, it just sort of appears between cuts. The implication was that it came loose over the film but up until that very shot, his face looked pristine.
Continuity mistake: When Barb puts Chucky down in the attic, when the rocking horse scares her, Chucky's face goes from being in the shadows to illuminated instantly between cuts.
Continuity mistake: Barb turns the flashlight onto the child's wheelchair twice between cuts in the attic.
Continuity mistake: Right after Nica and Barg get into an argument over the doll, Nica's feet go from together to about 6" apart. The character is in a wheelchair and can't move her legs, so this makes no sense.
Continuity mistake: After Nica gets into an argument with Barb over the doll, Nica pulls out her phone and then slams it down into her lap. Immediately in the next shot, the phone vanishes.
Continuity mistake: When Chucky kicks the bucket over, not only can you see the water hit Jill's feet twice between cuts. And both times, the way it hits her feet is different, revealing it's two different takes. (The water is more forceful the first time, while the second it's a bit "softer.")
Continuity mistake: When Barb and Jill are flirting over a video call, the second time Barb tries to signal Jill that she thinks Alice is up, her hand gets much closer to the laptop between cuts. (In the first shot, it's up near her shoulder; in the second, it's right over the laptop keyboard.)
Continuity mistake: When Nica and Barb are talking after Alice gets Chucky back, there are several minor back-to-back continuity errors as it cuts between Nica's closeup and the side shot with both women. Nica's mouth goes from closed to open and back a few times. Nica's arms aren't folded, then it cuts to the side shot, then it cuts back and they're folded. (We would have seen her move to fold them in the side shot.) And the direction she's facing changes subtly at least once.
Answer: It is revealed when Ian checks the webcam that Chucky hid Alice in the closet. She was most likely there the whole third act.