The first time reaching this battle, I worked my way around to the turret control terminal without entering the rotunda. I "peeked" into the Rotunda just enough to see a little bit of the battle going on (saw the Behemoth for sure as it registered on my HUD). Well I hacked the computer to change the turrets targeting parameters, and it seemingly killed everything in the rotunda and the behemoth was MISSING. I checked all the adjoining areas and no behemoth or it's corpse.

Coincidentally I saw something similar just a few days ago.

I did the same thing got to the turret computer and activated the turret. The Behemoths body was way across the Rotunda in a doorway directly opposite the turret computer room doorway. There might have been some clipping problems with something as big as a Behemoth and given the irregular rubble there (as I recall) so it may have gotten "buried" in the rubble in that doorway.

BTW, thanks for correctly identifying the building as the Capitol Building and not the White House, as so many others have in other threads