Analysis:What Caused Satoru to Drown

On the 1st day of Satoru's route, it's possible for him to drown.

Although Kokoro's consciousness took the sleeping pills, she doesn't see the consequences of her choice. Inubushi exhibits strange behavior behind the scenes, and Kokoro really does die if she doesn't take the medicine.

These mysteries were never answered.

Sorting out the scenario
Although surveillance cameras are installed in every room, none are installed in the bathrooms. In summary, they give the following information:
 * 10:48: The pills are consumed (or not consumed, depending on your choice)
 * A warm bath is prepared
 * A sound is heard in the room
 * Kokoro sees nobody when she pokes her head out of the bathroom
 * The sound is heard again
 * A proper search of the room confirms nobody is in the room
 * The lights go out
 * If the sleeping pills weren't taken, she feels the presence of someone entering the bathroom
 * 10:57: Transfer (Kokoro's consciousness leaves Satoru's body and α's enters)

If the sleeping pills are taken:
 * 11:19: The camera shows Inubushi in the room acting strangely
 * 11:30: Transfer (α's consciousness leaves Satoru's body and Satoru enters)
 * Satoru awakens in the bathtub
 * 11:33: Inubushi leaves the room
 * 11:37: Satoru exits the bathroom

And if the sleeping pills are NOT taken: (By the way, although it's not mentioned in the game, Kokoro's consciousness doesn't notice the person if she took the sleeping pills simply because she was sleepy)
 * The situation is completely unknown when α's consciousness is in control
 * 11:30: Transfer (α's consciousness leaves Satoru's body and Satoru enters)
 * Satoru drowns in the bathtub. This time, someone held his head down.

Two questions
We see two strange points from the above summary.

First, Kokoro's consciousness hears two sounds. Although the sound came from the room, there were no signs of life. The surveillance cameras also recorded nothing in the room.

Second, the person who turned off the lights and entered the bathroom. Who was it? Was it Inubushi, who was recorded by the camera about 20 minutes afterwards.

The source of the sounds
Kokoro's consciousness heard the sound but couldn't see anyone. Even after look for someone hiding, she couldn't find anything. Still, the sound came from the room.

Since the sound is heard even when not under the influence of the sleeping pills, it's very unlikely that she misheard it. In that case, it must have a definite source.

Did someone knock? Was it an echo of a sound from another room? Changes in atmospheric pressure? The snowstorm outside? Snow fell off the roof?

There are a variety of possible causes, but there is a most likely cause.

It's a rat.

Even if the rat makes a sound, she wouldn't find anyone because the rat is not a person.

Although rats were heard in the basement of SPHIA, there was one on the first floor that died due to the MAO inhibitor. So it's quite possible for them to move throughout SPHIA.

Security Cameras
The surveillance cameras that monitor the room should have recorded the person that entered the bathroom and turned off the light. Yet no such recording is found. The reason is that although the camera is installed near the room's entrance, it doesn't record the area near the bathroom door.

While Satoru was looking at the surveillance cameras, it says "Someone's back appeared at the edge of the frame. It was Hotori." If the surveillance camera was pointed at the entrance of the room, we would have expected him to say something like "the door opened".

Furthermore, the surveillance camera CG shows they only record part of the bathroom door. Based on the structure of the room, it's possible that a single camera doesn't cover the entire room.

Regardless, the camera barely covers the area near the bathroom. If the camera recorded the person who came in, they would barely be seen in the corner of the screen. Even if it hypothetically saw them, it's not possible for the person's whole body to be recorded.

Identity of the person just before the transfer
Just before the transfer, someone entered the bedroom, turned off the lights and came into the bathroom. Who is this?

It is probably Inubushi (Hotori's consciousness). It's natural to think of her when you consider the way Inubushi wanders around the room after exiting the bathroom.

Identity of the culprit of the attempted drowning
What's left is to uncover the identity of the person who tried to drown Satoru.

After the first transfer, ω (the female fetus) takes over Inubushi.

If Satoru took the sleeping pills, his body would be tired and simply remain leaning against the bathtub unnoticed by ω. ωleft the bathroom, was recorded by the camera behaving strangely, transfered consciousness and left the room. Satoru's body would remain sleepy and probably submerge its head in the bathtub it was leaning against.

But if he didn't take the sleeping pills, the consciousness that takes over Satoru would move about, attracting ω's attention and resulting in her pushing his head into the bathtub many times. When Satoru transfers back, Inubushi would continue holding his head under, resulting in him drowning.

That's the basic idea, anyway. This is only one possible scenario, we can imagine plenty of other scenarios that occurred in the bathroom.

By the way, the reason Hotori came to Satoru's room would be to thank him for calming her down when she went berserk earlier. Perhaps she turned off the light because she panicked, thinking the man was naked.

Summary

 * The source of the sounds was likely a rat
 * The cameras did not capture the area near the bathroom door
 * Hotori came to thank Kokoro for calming her down earlier, but turned off the bathroom lights in embarassment when she realized Satoru's body might be naked.
 * A transfer occured. One fetus controlled Satoru's body while the other controlled Inubushi's body.
 * If Satoru's body consumed the sleeping pills, it would not have moved much and ω would have left it alone. Satoru's body slumps and falls into the bath. After transfering Satoru is able to get out easily and Inubushi leaves the room, caught by the video cameras.
 * However, if he did NOT take the medicine, he would have moved and attracted ω's attention. Out of ignorance, ω would have pushed Satoru's head underwater. After transfering, Inubushi continues holding Satoru's head underwater until he died.