He had seen them come out and get into the unmarked police car.

他看见他们出来了，进了没有标志的警车。

《柯林斯英汉双解大词典》

In fact it's an unmarked police car.

其实，这是一辆便衣…警车。

danci.911cha.com

We were off duty, but driving an unmarked take home police car with hidden lights and sirens.

我们已下班，但驾驶无标志“带回家”警车， 有隐藏式警灯和警笛。

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