As Andy has said, there are various places where the PAS fluid could be lost.
For example it could be low-pressure PAS lines, high pressure, or the steering rack. I've seen them all go, by the way.
If you're unlucky, the fluid came out of the tx box breather pipe, next to the expansion tank. Now that's worst case scenario, meaning your tx box is Kaput (ram seal burst) but you don't know that yet, do you?