Okay, so a few days ago, some guys in the Rockstar Games Social Club found some code files called "vehicle.ide", the same code used for GTA vehicles. These codes contain information about the vehicles.
Max Payne 3 has more around 100 codes inside it, about cars, bikes, boats and helicopters. Some of this vehicles can be seen in the game, but the most of them can not be found. That led to many fans to believe that this is a hint to Grand Theft Auto V.
Here is a video shows that information. Credit to GTA-Series.com.
If those vehicles will appear in GTA V, I think we will have a lot of fun.