Parking garages can be treated as a separate building in a number of ways, by being a separate building accessed by a walkway, by being separated by a firewall, or through the use of a 2 hour horizontal fire separation (this is a unique allowance in the code).
In the first instance with two buildings being connected by a walkway, two separate fire alarms are to be provided, however, it is best practice for those alarms to communicate. There has been some movement to codify this requirement, but I'm unsure if they have made much progress yet.
In the other two situations, the buildings are to be treated as a single building for the purposes of the fire alarm, unless there are no openings in the firewall.
The whole concept they are trying to accomplish is to prevent people from moving from a safe area to an unsafe area by providing notification of the fire condition.