Reminds me of one of Terry Pratchett’s characters who went to the school of “My dad says”, the college of “It stands to reason” and was now a post-graduate student at the university of “Some bloke in the pub told me”.
That’s an excessive generalization. Undercover cops might need to deceive, but most cops aren’t undercover. Saying “Without deception, the police couldn’t do their job very well” sounds like you’re talking about all police.
Even when not acting undercover, police regularly use deception in order to get cooperation from people they feel would otherwise be cooperative. There are many situations where police are legally allowed to lie to those they have in custody, and they tend to lie frequently in all of them. Even traffic cops regularly outright lie or omit truths in order to catch criminals (a common omission being “do you know how fast you were going,” which they use to get people to admit to speeding when they see someone and think they were going to fast, but didn’t actually clock them with their radars to confirm it, you can often get out of speeding tickets by just looking at the officer and saying “no, I don’t know, how fast was I going” or by just saying a number under the speed limit when asked).
Although reading the comic and then reading my response again, that really only applies to the general case of “police being deceptive” and the specific case of “police claiming to not be cops.” Claiming to not be a cop is literally only useful when undercover, yes, because by definition a police officer becomes undercover when they start trying to hide the fact that they’re a cop.
I think a better response would be:
“Do you know how fast you were going?”