HSK 7-9
不翼而飞
búyì’érfēito disappear without trace; to vanish all of a sudden; to spread fast; to spread like wildfire
桌上的钥匙竟然不翼而飞。
Zhuō shàng de yào shi jìng rán bú yì ér fēi.
The key on the table unexpectedly vanished without a trace.
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Appears across levels (7-9) in the HSK 3.0 word list.