It seems fairly clear to me how it could come about. Someone writes a sign "Honking the horn at me is OK, please warn me as I don't have mirrors", which is cle

It seems fairly clear to me how it could come about. Someone writes a sign "Honking the horn at me is OK, please warn me as I don't have mirrors", which is clearly too wordy. And so it evolves over time, is shortened, newspaper headline style, via e.g. "horn is OK, please warn" to the final product. "doesn’t seem to make grammatical sense" well yes, but neither do many headlines e.g. "Diana Dead" that drop implied words such as "is" for similar reasons.