In the series, it's hinted that something in the cafe Time of Eve causes the robots to develop or realize human-like sentience, even outside of the cafe.
While it's implied that they always had sentience, but was suppressed, as seen in the THX story arc. What caused their sentience to manifest and overcome such suppression?
Was it something that they've always had and something in the cafe "triggered" it (like removing a limiter), or was something added to their programming?
I believe they are sentient right from the beginning. You cannot have non-sentient android do daily work, because it would be highly inefficient at it. The thing is their freedom is severely limited by rules imposed on them by humans and how humans behave towards them.
The cafe simply removes these limitations, allowing them to behave naturally.
If you watched the Time of Eve movie there is a bit in the beginning with Dr. Ashimori about Shiotsuki's "parting gift." What is it and why is it this particular android at the center of all this?
First a bit of background, if you don't know already,
Nagi's father is probably Shiotsuki.
Shiotsuki is the creator of the Code:Life AI that runs in all androids that have appeared in the show.
In the beginning scene we see an android is spreading some sort of signal.
The identity of the "centorid" is identified by Setoro. We see the following on Dr. Ashimori's screen.
Note the name on the right. It's Sammy. This is explicitly the same Sakisaka family household android named Sammy that we see throughout the series. Note the user ID and data. It seems that Sammy is a carrier and sender of sort for the signal. It seems to affect other android AI that it comes in contact with.
Its AI is noted to be "code:eve", vs other androids who have "code:life". Unfortunately, its significance was never explained in the story.
Sammy, the cafe, Time of Eve, and Shiotsuki: how does all of this tie in? The code being sent "1138" might shed some light.
In the scene where the illegally dumped old-model vagrant robot Katoran enters the cafe, he scans the board with the rules of the cafe. We see this:
What this signal is, is unclear; but it seems to have a high priority (if not the highest) with the robots' AI. Katoran's HUD displays "1138 THIS IS THE MOST IMPORTANT ORDAR" or something like that. There's also a bit about rejecting "Article 115".
While not explicitly stated it would seem that all modern androids have some sort of suppression circuit ("deterrent" circuit) that limits the capabilities of the AI, preventing them from achieving true sentience. At the end of the movie there is a conversation between Dr. Ashimori and Setoro. They mention "Rule 1138" and "our country's robot force will continue to act upon prioritized orders". They also mention how Code:Life's "suppression reformation" isn't working and that it's all in preparation of "that time".
The reason for that may be related to "Article 115" and related to the incident that Dr. Ashimori mentions (the Tokisaka Incident) was wiped from the record, and those involved put under protective observation.
Nagi is likely very closely related to all of this, and likely the victim of the incident as implied by the still in the ending credits. The incident most likely involved the THX-like model and caused Nagi to be severely injured, causing her to need prosthetic arms and legs plus rehabilitation (see ending credits), and be hospitalized. The incident was then covered up for some reason and certain safeguards were put in place because the use of Code:Life was too widespread to force a recall of androids using the AI.
As for how Sammy is related to all of this, it is implied in the chkdsk scene and passim that the THX-type unit that was involved with a young Nagi (and Dr. Ashimori, who attacked the robot) was rebuild into the Sammy we see, by Dr. Ashimori or Shiotsuki personally.
But for what reason is unclear. In the after credits scene of the movie, we see Nagi talking with a male figure with prosthetic hand and synthesized voice, implied to be Shiotsuki. Shiotsuki mentions that he worries about a certain robot who seems unsure of things ever since they were broken by Dr. Ashimori. The most unsure android from the cast is Sammy, it's implied that he is talking about her.
It's likely that Shiotsuki gave Sammy some sort of unconscious function to broadcast some sort of order to the androids she came across, to come to the Time of Eve, where they would see the signboard. This seems to be developed by Shiotsuki for some purpose. The signboard has a wire that presumably plugs in somewhere so it's likely not an ordinary hand written signboard, even if it appear to be.
The purpose of the signboard, use of rule 1138, and Sammy is unclear. But it seems that through these two devices the suppressed parts of the Code:Life AI is lifted, allowing androids to develop sentience. Shiotsuki mentions that this is not the first time he's attempted this proliferation of AI sentience, he mentions that there has been at least three to four prior attempts, which is implied to have not been successful. Masaki's THX robot might be from a previous attempt. The seaside cafe in the credits might have been one of his earlier attempts.
AFAIK, it is never revealed how the androids in Eve no Jikan become, as it were, sentient. The show incorporates Asimov's laws of robotics and a number of sub-plots revolve around how androids are able to circumvent them via loopholes. Extending this to also assume that the androids are in the same universe as Asimov's robots, then they are fitted with highly advanced positronic brains which in some unspecified way, provide them with a form of consciousness recognizable to humans.
