Let's just hope they have a system that directs you to where empty spaces are...
I remember the parking garage at BWI airport has sensors over each parking space. When the space is empty, it shows a green light that can be seen from far down the row. When someone parks there, the light turns red. There are then signs at the end of each aisle showing how many empty spaces are in the row, and a master sign as you enter the garage showing how many spaces are available on each level. No need to go hunting.
A garage I use here in Boston has the available spaces per level totals, but as it doesn't have sensors on each parking space I presume it just uses sensors to detect when a car enters and leaves each level to subtract and add numbers as necessary.
-Rob