Actually there are 4 fuse locations in a W210
- Main under hood - left side rear corner - misc items in the front of the car
- K40 relay in the electronics box - right rear corner under the hood (the box that says "battery under rear seat") - engine electronics
- Sail panel on left end of dashboard - exterior lights
- Under right rear passenger seat by the battery - misc items in the rear of the car

You're on the right track with the meter in series, sounds to me like the "573" is 0.573 A or 573 mA. That would be about the right amount of drain to pull the battery down 0.4 V overnight.

You can rule out stuck glow plug relay or ABS pump relay, those draw way more than 573 mA.

Any electrical modifications to the car?

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech