You can find secret messages on the Special Edition version of the paper pack-in GTA 5 map, which can be seen only with a UV (black) light. The lengthiest message reads:
T.P.E - Trevor Philips Enterprises. Weaponry, narcotics, intimidation and casual wear. Lago Zan Cudo. Great Ocean, Highway north, through north Chumash, 0.5 miles past Raton Canyon. When guard rail ends, follow the dirt road down the docks. Bring $500,000 cash.
Going to the location indicated by the secret message takes the player to a remote dock available for sale. The Minisub is stored there, and offshore a number of Nuclear Waste barrels can be gathered for cash. Other messages can be found on other parts of the map referencing aliens and the Buzzer and Lazer aircraft. Going to the location indicated by the flying saucer reveals a very well concealed Spaceship Part.