Hydraulic fluid o w140?

[MPM77]

Hey guys, I just had my steering pump and steering box replaced and I was checking the fluids under the hood and I noticed that there is a reservoir for hydraulic fluid (located on the drivers side in front of the shock tower under the plastic cap in a semitransparent reservoir). Car in question is a 98 S500 with level ride but without ADS. I thought the hydraulic fluid was for the steering system (tandem pump), but while checking the fluids the steering fluid was ok, but the hydraulic fluid was bone dry. Did the tech forget to refill? Or is the hydraulic fluid for the rear leveling suspension? I don't see any leaks at the rear of the car, so why is the fluid low? Are the accumulators full of fluid? To refill, do I simply remove the dipstick and refill through that opening? I have read (in this forum) that if the accumulators were bad (i.e. fill of fluid) that I would be experiencing a very bouncy ride, however, the car rides beautifully.

All replies are much appreciated!