I did try to mess with that coded banner text, but there seems to be no shift cypher. (Thought it might be a shift of three.)
Surprised that someone else considered the “Watase --> Root Double” connection too, lol. I wouldn’t necessarily say that that Watase is “cool”, though. If we still consider roots, though…
3 is the square root of 9, cubic root of 27 (the 3 might be the degree of the root). The —
holy fucking shit
ahem
Although the door was “opened” for the medics, that does not require that the door be unlocked personally by the victim! The fourth time the door was unlocked being “as mentioned above” does not refer to the door being open for the medics to come in in the aftermath of the stabbing! The victim locked/unlocked the door for the fourth time before the door was opened for the medics!
D unlocked the door with their extra key for the culprit to enter the room after the victim turned in for the night and locked the door! After the culprit stabbed the victim in the leg, the culprit exited the room and D locked the door behind them!
Nobody else knew that the victim normally woke before daybreak! Hence, no one questioned him waking up after sunrise! Some trivial circumstance kept the victim from using the phone to call medics before daybreak, such as the phone being out of battery or the victim falling back into unconsciousness due to the wound! This is the truth behind the belated call for help!
There is a hole or other method of transportation in the door or in the room that let the attacker stab the victim by dropping or throwing the knife from outside the room!