Go to Table of Contents of the analysis (which has also a statement on purpose and manner of analysis and a disclaimer as to caveat emptor and my knowing anything authoritatively, which I do not, but I do try to not know earnestly, with some discretion, and considerable thought).
The most important thing which must be kept in mind with Kubrick's films is there is the surface or principle story and then the internal or sub-story. In many of his films, if we're really paying attention, set elements pretty much immediately destroy the surface naturalism. One may not notice this destruction the first, second or third time one watches the film. Through constructive disorientation and disconnectedness, and sleight of hand as to where our eye focuses, Kubrick, the magician, intentionally obfuscates these elements that destroy the overt and naturalistic story line. The surface story lines are the principle ones, and this is maintained and supported by the intentional obfuscation of the deconstructive elements which keep them sub rosa. At the same time, these deconstructive elements are plainly there, alongside his tremendous effort to make things look real and believable, and once we bypass the disorientation and his purposeful refocusing they become a puzzle, annihilating the sense of reality. This destruction of the film's naturalistic story line is difficult enough to conceive of and accept that most people stop at this point and decide these puzzling aspects of Kubrick's films are errors when they are not. They are part of the art of a director cleverly designing the overt story line to be unimpeded by an internal story that tears it apart. Indeed, the sub rosa elements of the internal story may be discreet but they are enough in evidence to complicate the surface story with an aura of attractive, indefinable mystery, which is one of the reasons viewers return to Kubrick again and again. To work with the "reason" and "why of the internal story line is to try to settle into Kubrick's sensibility, examining how these internal stories form a dialogue in his oeuvre with repeating themes and ideas, elaborated upon from film to film. The internal stories haven't a "plot"; they aren't that kind of story. Instead, you have to be willing to deal with comprehending the themes and ideas represented in them as instead ultimately forming a different terrain for the setting of the surface story, guiding and interacting with the overt story and giving it a new form.
The Film's Internal Logic for the Discovery of the Mask on the Pillow
As to the *how* of the mask resting on the pillow. It has been there all along, at least in the logic of the movie. The first time we saw Bill's office, the second night after the party, there is no cabinet against the right wall in his office beyond the living room. Instead, there is an easy chair and a cabinet in the corner that is taller than the chair rail.
Then, later that same night (early in the morning), when Bill returns home from the "orgy", he places goes to the office where there in place of the tall cabinet and the easy chair there is now a long, low cabinet, the top of which is just below the chair rail. He first approaches the cabinet by bending over it and touching it, rather than crouching.
He removes an accordion folder from the cabinet, ostensibly making room for the bag, which he then tucks into the cabinet and locks it.
As Bill is placing the bag in the cabinet, Kubrick begins a crossfade to the bedroom.
The crossfade merges the cabinet with the headboard of the bed and has Bill, in effect, placing the bag holding the mask on his bed. Note how the lines of the folder atop the cabinet form also a kind of duplicate headboard in conjunction with the corner of the room fading in.
The perspective lines of the folder fit in with the headboard of the bed and the picture frame.
The perspective lines of the cabinet fit in with the perspective lines of the shadows from the blinds.
Here we see the crossfade completed.
The camera now pulls back as Bill comes around the bed. Rather than simply lowering himself and sitting upon it, his actions replicate those in the office, as he approaches the bed by bending over it, placing a hand upon it. The thing is, he didn't have to bend over the cabinet, place his hand on it, then crouched before it. He could have simply crouched before it. Again, in the bedroom, he didn't have to first bend over the bed and place his hand upon it (as he had done with the cabinet), he could have simply sat down. The connection with him before the cabinet is being reinforced. Compare with the crossfade and he has been placed approximately in the same area as Bill in the office within the frame of the bedroom door.
He then seats himself and Alice awakens.
When Bill enters his room the third night, he sees the mask on the pillow.