What is missing from the apartment when Victor and Henry return?

The creature is missing from the apartment when Victor and Henry return, much to Victor's delight.

I walked with a quick pace, and we soon arrived at my college. I then reflected, and the thought made me shiver, that the creature whom I had left in my apartment might still be there, alive and walking about. I dreaded to behold this monster, but I feared still more that Henry should see him. Entreating him, therefore, to remain a few minutes at the bottom of the stairs, I darted up towards my own room. My hand was already on the lock of the door before I recollected myself. I then paused, and a cold shivering came over me. I threw the door forcibly open, as children are accustomed to do when they expect a spectre to stand in waiting for them on the other side; but nothing appeared. I stepped fearfully in: the apartment was empty, and my bedroom was also freed from its hideous guest.