We all know that Fred and George passed on the map to Harry.
But what happened to it afterwards? Here's what I think occurred...
Epilogue:
The year 2015, Harry Potter's house...
James Sirius Potter carefully listened as he tiptoed towards his father's office, where he knew his dad kept all his important papers. After making sure no creaking noises emanated from either the floor boards or the door, he silently entered the room and began methodically searching every drawer and shelf. He found a sheaf of official looking papers, a bunch of notebooks, and a whole collection of both interesting and boring things, but none of them were what he was looking for. After carefully placing his father's rather dusty Order of Merlin, First Class back into its bottom drawer, he cautiously retreated as quietly as he had entered, promising to himself he'd find it tomorrow.
The next morning, after blearily gulping down breakfast and waving goodbye to his father before he left for work, James went to look for his brother Albus. He found him sitting on the back porch, reading 'Hogwarts: A History ' . He sat down next to him, and spoke with the assured knowledge of his one year's experience at school. "So you're already preparing to go to Hogwarts, little brother? Just be careful. I've heard that the Slytherin Common Room is in a creepy place. Right under the lake if you can imagine!"
Albus Severus Potter slammed the book shut and glared at his brother. "I am NOT going to be in Slytherin!"James had seen that very expression on their mother's face a wide variety of times, but only when she was about to explode. He hastily changed the topic, realizing that if you wanted someone to do you a favor, you didn't go about it by riling them up. Clearing his throat, he hesitantly broached the reason he had come looking for his brother in the first place. "Er, I need you to keep Mom occupied for a while downstairs. For an hour maybe."
Albus just stared at James. "And why would I do that?" James fidgeted a bit and spoke quickly, "I have to look for my birthday presents!" Albus gave him a look that said 'I don't trust you one bit but I'll still do this for you' before abruptly grinning at his brother. "Done. In exchange for half the fudge Grandma Weasley gave you last week. And the Chocolate Frog card with Agrippa on it."
James shook his head at that. "The fudge is fine. Not Agrippa though, I've only got one of him. I'll give you Ptolemy instead, I know you don't have him either." Albus slowly nodded and as James got up and was about to go inside, said, "I know you're lying, you nitwit. Your birthday is months away. You'd better tell me what all this was about later." James just shook his head and went inside, waiting for his brother to do his part.
Ten minutes later, James sneaked into the kitchen to see his mother and Albus engaged in a deep conversation about the professors and Hogwarts itself, his sister Lily listening with wide eyes. He wanted to join in but he walked away noiselessly, and went upstairs to search the one place he hadn't yet, his parents' bedroom.
He carefully searched the room, leaving nothing untouched. But he couldn't find anything. He sighed, and sat on the bed, head in hands. Where could it be? Teddy had told him that Uncle George had confirmed the fact that Harry still had it, before the boys went on to dream about the mischief they'd achieve with it. He fell back on the bed, looking up at the ceiling only to see... what appeared to be a trapdoor almost hidden, painted as it was identically to the rest of the room. James grinned and after dragging the step ladder to the bedroom without anyone noticing, he opened the trapdoor and clambered into a tiny attic.
There was nothing much, just a trunk covered with a slight layer of dust in a corner. It was unlocked. James opened it, and found a lot of stuff- his father's old red-gold Quidditch robes, some textbooks, and more relics from his father's days at Hogwarts, even a Golden Snitch. He would have loved to go through everything, but time was running out. He reached the bottom and felt his hands close around a thick piece of paper, and his heart skipped a beat.
Reverently, he pulled out a blank faded parchment and spread it out on the floor. Touching his wand to it, James whispered, "I solemnly swear I am up to no good", and waited, just as the twins, and his father, and before them, Moony, Wormtail, Padfoot and Prongs had.
Before him, neat lines of black rapidly crossed each other and formed a sketch of the castle he already considered a second home. And right in front, was permanently inscribed the legend,
"Messrs Moony, Wormtail, Padfoot, and Prongs
Purveyors of Aids to Magical Mischief-Makers
are proud to present
THE MARAUDER'S MAP"
The End.