With a completely full inventory, I opened a box of ammo and the paper turned invisible and then went into my hands. Afterwards I was not able to pick anything up, because I was unaware that the paper was in my hands. A few minutes later, I must have bumped the mouse scroll wheel, because at that point, I received a prompt to, "Read Note?" I was then able to drop the paper from my hands and pick up other items.
Similarly, when opening a can of food, the can turns invisible and goes into my hands. The upside here, is that I get a prompt to "Eat" right away, so at least there was not the same confusion as with the paper.