Merriam-Webster's Dictionary: "sleeper" --
4: someone or something unpromising or unnoticed that suddenly attains prominence or value, eg <the low-budget film became the summer's sleeper>
Translation: someone or something unpromising or unnoticed that suddenly acquires significance or value.
Another: "What Does Sleeper Mean in the Antiques World"
http://antiques.about.com/od/thewise...g/aa050106.htm
Notice how the vowel is pronounced in the word and hear the difference.