I'll start, but then I'll let then the specialists on homicide surveys speak.
We don't measure “missing”. We only measure “murdered”, which is a big distinction. Homicide is probably the type of crime for which the resolution rate is the highest because there's huge effort put into investigations. Especially when the perpetrators are acquaintances or spouses, they're much easier to resolve. But we don't have statistics on missing people, and until we find a body we cannot conclude that there has been a homicide.