There should be a make up/fill valve somewhere in the system. I've seen both auto fill and manual fill set ups.
If you have chemically treated water or glycol systems even the auto fill valves are valved off, so you know when you dilute the system. If you have a substance in the system that needs to remain at the right concentration, you don't want leaks in the system to cause uncontrolled dilution.
If you can find the systems expansion tanks or air separator, the fill valve should be in that area.