That is right. The purpose of the sign is to WARN the trespasser of a dangerous condition on the property.
And if they didn't trespass in the first place? (not a rhetorical question either)
Does a rotten board on a porch, which could cause injury from someone falling through, predicate the need for a sign?
The owner knows it's there and avoids stepping on it until it's repaired. He warns guests to his property.
Should he also warn the thief coming to rob him unawares?