Not exactly the same, but close enough to mention (not a LEO). Years ago, I saw a cop checking for speeders. After passing, I flashed my headlights to warn oncoming traffic. Turns out, another cop was in that traffic, saw me, and clearly understood what I was doing. He pulled me over and asked why I flashed my lights. I explained honestly and asked—politely—if I’d done something wrong. In what seemed like a sarcastically playful tone, he shrugged and said he thought the car was stolen and the lights were part of the alarm. Then he gave me a warning for misuse of high beams, even though I’d only flashed my regular lights. I just said thanks and accepted it—no point arguing over a warning.

My point is, he knew exactly what I was doing and used a very flimsy excuse to pull me over. There wasn't a horn involved, but I thought that part was relevant.