This is the 9821st most frequent Japanese word.
覆面
"覆面" means "mask" or "disguise."
覆面 here means 'mask' or 'masked,' referring to someone wearing a mask in the context of a robbery.
覆面の男性が銀行を襲った。
A masked man attacked the bank.
覆面 in this case modifies 'patrol car,' meaning 'unmarked,' signifying that the identity or markings are intentionally hidden.
覆面パトカーが高速道路を巡回していた。
An unmarked police car patrolled the highway.
覆面 here refers to concealing one's identity, with '覆面作家' indicating an author who uses a pseudonym or hidden identity.