CV3 has a lot of empty placeholders in its code. If you open reVamp, go to Death's room, and right-click on Death's spawner, you'll notice in the menu that the slot between Death and the Mummies+Cyclops spawners is empty. All the data's been cleared (zeroed) except the y-value, which for some reason is always set to $20 by default. This aspect and the layout of the spawner data in the code suggests there were a lot more enemies or other NPCs intended, or that originally the interactive terrain was to be coded with the same spawner mechanic as enemies.
Anyway, the empty spawner before Death's didn't get reinitialized like all the other enemy spawner data. All spawners have a designated set of code they perform each step, just like phase counters in enemies (read through my blog for more details). Every boss in CV3 has the same spawner code which prevents the boss from reacting until Trevor is within a certain distance of it. That empty spawner next to Death's spawner is set to the boss code., meaning at some point in development there was another boss planned and removed. But like I said, the rest of the data's been reinitialized, so trying to spawn the deleted boss yields nothing. EDIT: Actually, trying to spawn the deleted boss sets the Boss HP to 8 (2 bars), which is the HP for enemy object 0 (due to the rest of the data being zeroed).