My husband was an abalone diver back in the 1960’s... he had friends that set lobster traps and found that someone was stealing from their traps. He said they greased the lines on the traps with a thick blue grease so anyone dragging them up to their boat deck would have blue marks on the side of their boat. They caught the culprits.
Weeeellll now have to give some thoughts to that.
Great story! I love to hear about cleverness defeating treachery!