According to Emily Smith and David K. Li of the New York Post's Page Six, a source indicated Harvey and some friends were out at the 1 Oak nightclub into the early-morning hours.
Harvey's no-show for Saturday's game resulted in a three-day suspension without pay.
According to Mike Puma of the New York Post, sources indicated Harvey told the Mets he was experiencing migraines Saturday after playing a round of golf earlier in the day, which is why he could not report to the ballpark.
Per Jon Heyman of FanRag Sports, Harvey believes the issue between the two sides was a miscommunication, and he is expected to file a grievance.
In a separate report, Heyman added that the Mets sent security personnel to Harvey's apartment Saturday night to check on him after he told a Mets coach via text message that he was too ill to attend the game.
According to Puma, Mets manager Terry Collins laid out his expectations for Harvey upon returning to the team: "One thing he has to do—and he's done it in the past—he needs to address some guys. He wants to do it in a group, which is the easiest, I always think. Or if he wants to do it individually, he can do that. I'm just going to leave it where he's most comfortable because we've got to get this behind us, so however he wants to go about it to do that, I'll sign on for that."
A source told Puma that Harvey had been late arriving at games on "a few occasions" and was "fined $500 at least once."
The Mets are struggling through an up-and-down season as they are 15-16 and trail the Washington Nationals by 5.5 games in the NL East.
Harvey—a former All-Star—is coming off a 2016 season that saw him go 4-10 with a 4.86 ERA, and he is just 2-2 with a 5.14 ERA so far this season.