Mob spawners have no mobs in them

I found two mob spawners that were working fine, one was a zombie, one was a spider. I marked the area down to come and make a grinder out of it later. I came back a few hours later to find that the small spinning version of the mob inside the cage is gone and not spawning anything. I've never seen this happen and I would like to know if this is a bug or something with my world. I hope it's a bug.
And no, I'm not going to go in creative and put a spawn egg in there.