Surely every plain clothes officer who's out on patrol (as, for example, undercover officers are on Oxford Street, watching for pickpockets or their colleagues are in airports, watching for drug smugglers and terrorists) is pretending not to be a police officer, isn't he/she?
Similarly those police officers who post on social media, pretending to be children, in order to catch those who're trying to involve children in sexual activity are also 'pretending not to be police officers'.