So even a system that never sees rain can collect water?
As others say, YES. Here's a little physics for you.
There is moisture in the air. The air is the reservoir is about oil temperature. The top of the reservoir is about room temperature. The moisture condenses on the inside of the top of the reservoir. The water drops into the oil and settles in the bottom. Cylinders move exchanging air in the reservoir. More moisture. More water.
The company I once worked for built hydraulic power units. Some of our customers specified stainless steel tops on their reservoirs. There was still condensation and water, but no rust particles from the falling "stalactites" of rust.
There are solutions to help this (sometimes work) with desiccant breathers with usually WAY too little capacity.
There are what I'll call air bags which expand and contract sealing the reservoir. This concept was (AFAIK) pioneered by the Greer Accumulator company as Kleen-Vent. I don't know if Parker, who now own Greer, still offer them. IMO, that's the best way, but not always practical, rarely inexpensive.
At least 1 injection molding machine manufacturer incorporates them in some of their machines. There is an open (flattened expanded metal, usually) box with what looks like a bag made of Harbor Freight tarp material inside.