I know exactly what you're talking about - I've had that happen too. I turned my eyelids inside out trying to find the lenses in back of my eyeball, but to no avail. When no irritation developed, I concluded that normal explanations wouldn't explain it...

What I figure is that (being half asleep) I put them on the sink (a big nono). A cat then probably knocked it off the sink and into the toilet. Since I can't see anyway, I never turn on the light at night and probably flushed them.

Or the couch hid them somewhere...