Remus Lupin was sitting at the Gryffindor table, in the Great Hall, with Peter Pettigrew, but something was weird: James Potter and Sirius Black were missing. Indeed, at the beginning of the afternoon, James and Sirius had the "amazing" idea, as they said, to make an unnoticed trip to Hogsmeade, thanks to the secret ways showed on the Marauder's Map. Remus didn't want to do it, he had prefered to spend his afternoon studying for the future exams, and Peter, who had hesitated, had decided to follow Remus and to lock themselves at the library.
However, it has been about five hours that their friends were gone to the village and they haven't came back yet. Remus was starting to panic, he was asking himself where they were and what might have happened, if something awful happened to them, though Peter was pretty peaceful. The werewolf was thinking about a lot of hypotheses and he told himself that a professor had probably caught them while they were trying to go to Hogsmeade or while they were there. If this was true, the two men would have more detention hours and Gryffindor would lost some points.
The young Gryffindor couldn't wait more, he had to do something. He got up and left the Great Hall, without saying something to Peter who was already stuffing himself. Remus was trying to remember all the secret ways of the castle which will lead him to Hogsmeade and he walked to the closer one.
When he arrived in front of the way, he looked around to be sure he was alone. Nobody was here so the young man started to go into the secret way when he heard footsteps at the end of the corridor. The footstep were getting closer and Remus was panicking a bit because he couldn't see who or what was coming to him due to the poor light. Much to the werewolf's surprise, James and Sirius were almost running to him. The panic didn't leave him and he was starting to feel a bit of his anger coming.
"Merlin's beard, where have you been?!"
The two Gryffindors looked at each other before looking at their friend. James was biting his bottom lip and Sirius was looking away whereas Remus looked at them with an unbelieving gaze. He didn't understand what was going on. His friends finally spoke and told him what happened. Actually, they came back from Hogsmeade a few hours ago but Argus Filch was waiting for them at the end of the secret way. The caretaker of Hogwarts had noticed the parchment Sirius was holding in his hand and he was more than happy to confiscate it from them. Filch had took them to his office and had gave them a few detention hours. They got more detention hours in the end because they tried to take back the Marauder's Map.
Remus was more than shook, he was speechless. Even after knowing what happened, he couldn't believe how they lost their most valuable possession.
Then, the three boys returned to the Great Hall and told everything to Peter who couldn't help laughing, but he was also worried because of Remus' look. James and Sirius were trying to reassure the werewolf, telling him the map was just a parchment and that nobody could reveal its secrets, certainly not Filch who was a Squib thus has no magical power.
What the Marauders didn't know was that, much later, two Gryffindors had the "chance" to go in Filch's office and they took the map of which they easily revealed the secrets.