This was a friend of mine... She about had a heart attack when she couldn't find her. After about 45 min, they sent her upstairs to lie down because she was about to pass out...and when she leaned on the bed, the little girl woke up... she had been hiding behind the pillows on her parent's bed and fell asleep.
I dread this happening to me any day now because my little one is big into the whole hide-n-seek thing and she's getting really good at it.