Do it!: Classic Conundrums

During the war, several British naval spies posing as Canadian lobstermen were posted off the coast of Nova Scotia. Their mission was to be on the lookout for strange activities. They took great care to act like the locals, and even perfected their Nova Scotian accent. They had a number of fake red lob...sters made, and every night, the spies loaded the lobsters into traps and threw them overboard. At daybreak, they would hoist the traps aboard their boat, pretending to work while surveying the coastline. The locals spotted them as fraudulent lobstermen from quite a distance. How?