During a mining trip I fell into a lava pool holding a freshly made iron pick. Upon being damaged the pick disappeared, I did not die from this. I re-logged and it did not reappear. To test it again I went into a cave with a freshly made iron pick and diamond sword. I found a skeleton and upon being shot by the skeleton while holding the pick it disappeared, and then while holding the sword got shot again and it too disappeared. The disappearing happened when damaged by skeletons, creeper, fire/lava and zombie as far as I tested, spiders didn't seem to make them disappear. Disappearance did not happen every time damage occurred. None of the items have magically reappeared since their disappearance. In all of these events/tests I did not drop the tools.