I am certain it is under the spare tire on mine. Remove the tire and you will find a plastic cover in the center of the spot where the tire was. Remove that cover and you will find the hydraulic pump and reservoir. It is a strange place to hide it...

You may be seeing the ABS Pump that is under the hood. It is located right behind the Left Front Headlight.

__________________
02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)