Sleeper refers to outwardly common looking item that possesses something special or unique inside.

“潜伏者”指那些外表看起来很平凡但却蕴含奇特内在的事物。

Example:

That boring looking car is a real sleeper with that big turbo hiding under the hood and nary a sticker or scoop to be seen anywhere on it.

那辆看起来平凡无奇的车是个真正的“潜伏者”，引擎盖下装有巨大的涡轮增压机，这种配置上哪儿也买不到。