DCHP-2
ghost car DCHP-2 (May 2016)
n.
an unmarked police car (see Images 1 and 2).
Type: 5. Frequency — The most likely interpretation is that ghost car, which is reported first in a US context (see 1938 quotation) is a preservation of older American usage in Canadian English. The term is quite widespread in Canada. While ghost car has other general meanings, such as vehicles that cause bodily harm without making physical contact or vehicles that appear to vanish into thin air, among others, the meaning 'unmarked police car' is used nationally in Canada. Chart 1 shows predominant use of the form in Canada.
See also COD-2, which marks it "Cdn".References:
Images:
Image 1: Decomissioned ghost car. See photo caption (Source: http://www.intelligencer.ca/)
Image 2: Historical ghost car, on duty at UBC campus (source: The Ubyssey, 15 Feb. 1962)
Chart 1: Internet Domain Search, 19 Nov. 2015