Did Squire Trelawney keep the treasure map a secret? How do you know?

From the text, we can infer that the Squire did not keep the map a secret.

“I'll tell you what I've heard myself,” continued Captain Smollett: “that you have a map of an island, that there's crosses on the map to show where treasure is, and that the island lies—” And then he named the latitude and longitude exactly.

“I never told that,” cried the squire, “to a soul!”

“The hands know it, sir,” returned the captain.