On occasion I find that my health starts to decrease and I hear the zombie / husk sound. There is nothing around me. I equipped my shield and i noticed that it made the blocking sound and animation. I tried to attack the 'air' with my sword but nothing happened. I was playing with another player and I called them over to where I was located and they said that the mob was there. 

This happens quite regularly, but it came to a head today when I was exploring a Nether Fortress and I noticed that I had picked up withering. No skeletons nearby, nothing, in fact. Then, I die and lose all my stuff. The other player was not with me at this point, so I couldn't check with them.