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。

youdao

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。

youdao

Glenn Robinson's Citroen Saxo was being followed by an unmarked police car after being seen speeding between Horden Easington Colliery on Sunday.

BBC: Glenn Robinson killed in Horden car crash

At issue is whether electronic tracking is comparable to such routine surveillance as following a suspect in an unmarked police car on city streets, as the federal government argues.

WSJ: Judges Challenge Phone Location Tracking

We know that an officer in an unmarked police car had seen the car being driven at speed along Essington Way and turned to follow it with the aim of getting it to stop.

BBC: Glenn Robinson killed in Horden car crash