No matter what can or can’t be said of Mormon doctrines, not many can disagree that they are the premier record keepers outside of a few government agencies...and even that is debatable.
It is an absolutely safe bet that the Mormon Church has 99.9999% accurate centralized records of where every missionary is; where they are from; when they were sent; and when they are scheduled to return.
Yes- no doubt about it. Why they aren’t counted is mind-boggling.
Just spent a week with more than 40 LDS. Trust me, they know where their missionaries are and when they are coming home.
The simple solution to this would be for every Mormon household with a missionary to simply report him/her as presently living in the house. The house is, after all, their permanent residence. They don’t stay at any one location on their missions for more than a few months.