wow, I'm pretty sure I would be beyond freaking out if my cat swallowed something as big as an earplug (I mean, that just seems really big to pass through the cat's body normally?), but then, I don't have that much experience (only having one at a time) with cats I guess, so I defer to those with more. I'm increasingly thankful the weirdest thing my cat tries to steal from me is saltine crackers..
But, isn't it still possible that your cat batted it under the refrigerator or something? (my cat has lost so many small objects/toys under big furniture) I mean, if you didn't actually SEE him.