Why was Michael hiding in his sons car?

Michael knew about the financial situation surrounding the car. Consider how much he understood about Simeon's business practice, Michael obviously did his research. He also came from a criminal background, where he was used to dealing with other criminals as well. Remember that in the mission prior, Jimmy was just at the dealership purchasing the vehicle. This implies a very short time frame (same day or possibly next day) that the car was to be repossessed from the purchase time. Put two and two together, and it's not too much of a stretch to say that Michael took a good guess that the car would be a repo target around the time that Franklin took the car.

Maybe he was drunk and went for a nap in the closest thing to hand?