Here's the thing though: The police believe over a dozen people are dead. They need to track down and talk with a
lot of people besides Eva: relatives, servants who weren't on duty that day, people that had seen the victims recently, Kawabata (well they won't need to
find him since he reported the event and is probably cooperating), etc.
The police will want an accounting of everybody who was on that island, because they have precious few identifiable remains. Bear in mind it's a private island, so they can't just psychically know who was
actually there, and they may find evidence that shows merely that some people were
supposed to be there. For example, Ange was probably supposed to be there, but she wasn't, and the police would try to find that out.
Inevitably, they will ask the other servants who was supposed to be on duty for that weekend. The question is, what will those servants say? We can be all but certain that they'd mention Genji and Kumasawa, most of them probably knew Gohda was sure to be there... and... who? Shannon seems to have been Yasu's proper servant face, so they most likely would say she was supposed to have been working that weekend. Would they mention a Kanon at all? There's two scenarios here as I can see it:
- The servants do mention that a servant named Kanon was supposed to have been working with Shannon that weekend. The police investigate, and find... what, exactly? They're going to ask questions about who knew him well, when he came to the orphanage, when he started working, what his real name is and any documentation that exists about him, whether any known relatives were ever located for him, etc. They might even try to find his parents somehow if they're desperate for leads. None of that is going to turn up, is it?
- The servants don't mention anybody named Kanon. Just Shannon/Genji/Kumasawa/Gohda, all of whom are people with identifiable lives outside Rokkenjima. This would satisfy police investigation... right up until the message bottles claim there was another servant present. At this point the police go back to their witnesses, who continue to claim they've never heard of anybody named Kanon. The police should probably become immediately suspicious of the message bottles at that point.
But apparently neither of these things happened, or if they did, we know nothing about it other than the claim in ep8 that Kanon doesn't exist... which would rightfully follow from any investigation into him unless Yasu/Genji went to such ridiculous lengths as to forge paperwork for him, and even then there won't be anybody who knows him and perhaps most critically,
no school records whatsoever. Yasu/Shannon, remember, goes to school. I presume under her real name. People would know
of her and have seen her around at school, even if they didn't know her very well. This would be true of many of the servants, most likely. Kanon would have to live in the same places as Shannon and the other servants and go to school with them (or at least to a school in the area). Nobody at school will have ever heard of him. Nobody at
any school would have heard of him.
Again, how would this not set off red flags for the police? As far as they know, there may have been an uneducated, undocumented, mistreated slave boy on the island whose very existence has been wiped out by the explosion. Or else the message bottles are inserting a person who doesn't exist.