During my last work placement, our police often had to get something out of the ordinary for special duties like undercover /observation work during drug busts etc. It was no use using a car that was a normal government contract job, so we had an arrangement with local dealers to lend us something from their used car lots. No police equipment was fitted and the only equipment they took with them were portable radios to enable them to operate on the normal frequencies for our command. We borrowed Mazda 6's, Liberty's, I think there was a Beemer once. Pursuits were not part of the deal, they just followed the suspects at normal traffic speeds. As for how you would have spotted them as undercover cars, well, you wouldn't. That was the whole idea.
In relation to the classification of police cars in NSW, there were three general classification. Unmarked, which obviously had no police insignia or external equipment fitted, partially marked, which had police insignia only externally and fully marked, which had the full house of insignias and external equipment. The only real "undercover" cars were those like I mentioned that we borrowed. It was a common practice and probably still is. We had to take out insurance on the cars for the period we had them but that was about all. The vehicles were borrowed for perhaps one or two weeks, then went back and we borrowed another one, so that the crims really never knew what to look out for.