It is rather like the Princes in the Tower. For reasons I've already set out on the appropriate thread, I think Richard III probably had them killed, but it is odd, therefore, that the bodies were not publicly displayed (with claims that they died of natural causes).
I wonder whether Nicholas's body would have been displayed had he been killed alone, but then Mikhail's wasn't either.
However, it does seem to have been something of a Soviet habit to bump people off secretly, and leave the populace to wonder what had happened to them.
Ann