Additional Rules from the RulebookEdit
You can play this without another Treasure Map in your hand; if you do, you trash this and gain nothing. You have to actually trash two copies of Treasure Map to gain the Golds; so for example if you Throne Room a Treasure Map, with two more Treasure Maps in hand, then the first time Treasure Map resolves you trash it and another one and gain 4 Golds, and the second time it resolves you trash your other Treasure Map and gain nothing (since you didn't actually trash the played Treasure Map that time). If there aren't enough Gold cards left, just gain what you can. The gained Golds go on top of your Deck. If your deck was empty they become the only cards in it.
Secret History of Treasure MapEdit
Donald X. Vaccarino said this in an article http://www.boardgamegeek.com/thread/457023/the-secret-history-of-the-seaside-cards on Boardgamegeek.com:
This originally came from the ruins of the 7th expansion. I moved it just because of the name. How do you not do a Treasure Map in your Seaside expansion? The first version only gained you three Golds, and didn't put them on top. It looked cute but was not a real strategy. Putting the Golds on top helped tie it in with the next turn theme, and upping them to four made the card good enough to actually go for.