隐敝

tā men bǎo zhèng tōng qì kǒu hé guǎn dào huì yǐn bì de hěn hǎo jī hū kàn bù dào

他们保证通气口和管道会隐敝得很好，几乎看不到。

They convinced that vents and ducts and such would be barely noticeable.