Koppie signals and signs

Visited the koppie last night (Wednesday) for the first time. Alltogether 4 cars there at the same time. Obviously not all of them are doggers and thats a bit of a problem to know who to watch or to approach without getting your nose broken. The obvious things like switching on cab light or hazards or flashing headlights I did not see. Other areas have proposed feathers in hand or red items hanging from cab mirror...The thing is...how would one see that in the dark? I saw one car putting a bottle on its roof. Was that a sign maybe? Dont know. Any suggestions???