A few days after your first encounter with Jeff he'll call you asking for more help. Drive over to the marker then down into the underground parking lot.
Take Jeff's car over to the marker, but beware of cop cars, having a dead body covered in blood in plain view on the back seats will get you a wanted level if get close too close to a cop.
When you reach the marker follow the instructions to dump the car in the river.
Go to the barges moored by the shore just north of the corner of Grummer Road and Exeter Avenue. Use the stairs on the west side to get onto the barge then jump across to the next barge. Climb into the container and shoot the pidgeon.
Next: Wrong Is Right