Bingo !

The disconnect test is a good suggestion but OP needs the alarm on and be able to see the car via Mercedes Me otherwise OP would had went with standby mode the very least.

Agree with possible parasitic drain, when OP comes back a proper test like using a meter and start unplugging things might be due if (hopefully not for OP's sake) the car doesn't start upon return.

Disconnecting the battery has quite a few unfavorable consequences such as having to re-enter data such as entertainment presets, navigation destinations and points of interest, seat positions, ... Even dealers keep the juice flowing when installing a replacement battery.

I see.

and... interesting, how does that work?

I am puzzled by those setting losses. I have a 2014 W212, I have disconnected both batteries a few times such as I can do some electrical work. Because of my ignorance of electronics, I take both of them out just in case. I can break something mechanically, or toast something with heat but definitely not fry anything because of a short.

I have NEVER lost any of the COMAND unit settings, nor the sunroof, nor the seat settings, nor the convenience entry settings.

On the W166, I have never done it for fun because that main battery is a PITA to get out, and for the AUX in that back there are no words to describe it.

Yeah, don't think settings get lost when the battery is disconnected. The settings are not stored in volatile memory that needs to remain powered. It's persisted to non-volatile memory such as flash memory or harddisk in older cars. Just like in a laptop, you don't lose all your settings if you disconnect the battery or run it dry.

No user settings get reset with a battery change.

All user settings were reset in my '18 E300 purchased in July 2017 when the battery was disconnected for service and software was updated. Perhaps it was the software flash and not the battery change that wiped my profile. After that experience, I made a backup of the profile.

Happy to report the Kraut Wagon started right up after 7 days and a few hours parked at the FLL airport.
No Low Voltage warnings while I was gone, no nothing, all good.
Looks like the problem I had 6 months ago was caused by infrequent and short trips with the wagon parked (un-locked) in the garage for a week or two, and a concern about ghost drains, a possible short or a bad battery.
The jumper cables paid off: It never rains if you carry an umbrella at all times. (Old Chinese Proverb) 😎

Thats good to hear, much like you, I do not drive my car enough, I try to put my car on a battery maintainer once a week.

Good idea, I will keep doing the occasional battery maintainer despite the good news after my airport experiment.
Learned something from this thread and will put it to good use in the future: Standby Mode: Good for parking at home in the garage.

Good to hear all went well.

Still drive it occasionally, although doesn't happen overnight, I think flat spotting on tires will cause vibrations depending on how severe they are flat spot.

Good point, but flat spotting will probably take months.
How long does new, or even used cars sit on the lot without moving and without getting flat spots?

It depends on the tires and how severe the flat spots are. Minor flat spotting will correct itself. Certain tires also no longer have metal belts, so they are less likely to flat spot. However, it's more common than you think for new or used cars to have flat spots from sitting on the lot.

Maybe not months but depending on the tire pressure it might develop quicker than expected.

I believe they overinflate those tires, when the cars need to sit on the lot for a long while. And then air them down when it comes time to drive them.

Glad you ended up ok for the week. With my wagon I’ve parked it for a month with no issues. I had same year (2019) e450 cabriolet and that car would get low battery warming after being parked only 4 or 5 days. Never did figure that one out but got rid of the car quite sometime ago. Maybe it’s a software issue or something that causes the drain.

We parked the wife's coupe in the garage almost two months ago and it and it started right up today. Much more impressive than my BEV Audi with an 400v 86kWh battery backing up the 12v battery which was dead in the same time.