I can only think of 2 valid reasons for a low fluid level - a leak (which will need fixing) or a failed accumulator (in which case it will need a new accumulator).
Leaks can be internal seals, which are very hard to get hold of, or physical damage (split pipe, crack in reservoir, etc. caused by other work or accident damage)
The accumulator is a partially gas-filled sphere with a membrane. If the membrane fails, the gas leaks into the reservoir and the oil level drops by the amount that leaks into the reservoir.
They can be tested by removing and weighing.
As far as I am aware, there is NO other reason for a low fluid level. It is not a "sealed for life" system, but does not use or lose any fluid in normal operation.
The fluid does get discoloured over the life of the system, mostly from particles of seal. I recommend changing the fluid when replacing an accumulator, but I'm not sure that this is a required or recommended service step.
Just saying "it needs a top-up" tends to be mechanic-speak for "I don't understand it, have little or no idea how it works, and I'm willing to try something random to see if the problem goes away as fluid level is something I understand "
However, unless the level is checked using the proper procedure, the likelihood is that the system will be overfilled, potentially causing more damage.
