It's correct to say, "I found the map," (past tense) if one is directly reporting that event as it happened in the past, no matter how recent or distant. "I have found the map" (present perfect tense) indicates the map has just been found, or was found in the not-too-distant past; it conveys an air of finality.
Consider these two examples:
John combs the cellars, looking for the precious document. His week-long search has, thus far, proved abortive. Suddenly, he happens upon a chest half-buried in the dirt floor of a long-forgotten chamber. With some effort, John pries it open and, to his amazement, there lies the ancient scroll—the Map of __. Incredible! Unable to contain his excitement, John rushes out to find his mates, screaming at the top of his lungs, "I found the map! I found the map! I found IT!"
"It's absolutely impossible to get to the island. We don't know how to get there and the secret map has been lost for ages."
John turned around in his chair and said calmly, "There, you got it wrong Kevin. It is absolutely possible to reach the island." Measuring his words for effect, he pronounced: "I have found the map."
Everyone stared at him, dumbfounded.