"Go missing" is not a uniquely Canadian usage; it is also very common in British English (and indeed I suspect originated there). It seems to have taken a while to catch on in American English, however. For this reason objections to it have cropped up in American and some Canadian sources (people often object to new or unfamiliar usages simply because they are new or unfamiliar). Nonetheless, I think it is catching on in the US, as I have heard it used quite naturally on Law and Order and other American TV shows.
We have evidence of it being used for at least the last 50 years, and it is on the same syntactic pattern as "go astray", which has been in the language since the Middle Ages. The specific use of "missing" to mean "unaccounted for, not yet confirmed as alive, dead, or captured" arose in the 19th century. Its use increased greatly with the two world wars of the 20th century,and I suspect that this is why the phrase "go missing" arose during or shortly after the Second World War.
I suspect that "disappear" may have connotations of finality, and vanishing in a puff of smoke, and that is why "go missing" fills a useful purpose when indeed we do not know where someone is, or whether they are alive or dead. I cannot conceive of announcing to concerned relatives that their loved one had "disappeared" in a battle, for instance, and for the same reason in cases of abduction I don't actually think that "disappear" is better than "go missing". For that reason I cannot concur that "go missing" is "poor usage".
In any case, English is a language that has always loved synonymy, so just because "disappear" exists, that doesn't mean we should use it and only it.
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