There are several place fluid loss can occur.
Probably the most common is the steering rack leaking fluid into the rubber boots.
If the boots are in good nic and not cracked or perished, you will have to cut the cable tie from the end of the rubber to see if there is a leak there.
Replacement cable ties are common at many stores.
If you are only needing to top the power steering bottle up once a month or so, I would put a cap or two of brake fluid in as well.
The brake fluid will swell the rubber seals and usually stops the leak for some time.
There is also a stop leak power steer fluid available.