I had two cops on motorbikes do the exact same tyre thing, they said my rear tyres were worn below the limit. I just remained polite, hopped out of the car, found the wear indicators and pointed out that there was still a couple of mm left in the tread. It took a few minutes of the cop running his hand over the tyre, and looking at it to finally agree.
If you know nothing is wrong with your car, don't be intimidated, remain polite and point out the facts. If you are going to get out of the car, ask them if it is ok first as unless they ask you to get out they sometimes freak out if you just jump out of the car.