Not missing, not a mystery.
They drove their armored vehicle underwater and drowned. It's sad but stuff like this happens.
When I was in Korea, a similar occurrence happened to someone driving an M113 during a training exercise. When I was at Knox, two soldiers lost their lives when an explosion occurred during refueling. As others have noted, while it's tragic, it's something that happens from time to time in the military.
I'm pretty sure the president wasn't briefed about any of the events I mentioned either.