Hi again Ian,
I'm not sure if this has already been resolved somewhere else but I did have another thought...If we consider the meanings of the verb "missing", it is impossible for this to be a present participle. In terms of other adjectives not being used here, I resort to my original point about the coincidence of "missing" having a present participle form etc...
Certainly not important, but I had to spit it out
Retired English Teacher