The robots have attacked and dismembered him. You'll find they are very aggressive towards Calhoun's form, so I would suggest caution should you find parts of him.

The experiment must continue. [ Zero says nothing for a long moment. ] With or without Calhoun's body, once the virus is properly contained everything shall continue. Calhoun is not necessary for this facility to function, I am. Does that answer your question, Subject 3?