It’s probably only an aesthetic problem, but you should definitely contact Sonos and get another replacement.

You might want to post a couple photos here so you could point the sonos support folks to them if they have any questions.

Here you have an example of the right side

If that is brand new, I would definitely have it replaced.  

I don’t know the dust situation in your area, but this photo does not impress me as “new”.

Even if you’re ok with it and don’t want to deal with a replacement, I would let Sonos know.  It’s not the first time someone has posted a complaint like this.  I get the impression that there is an internal part that requires some sort of oil/lubricant that makes it’s way on to someone’s hands (or machine) that’s putting on the exterior shell.

The Arc has one week and the serial number starts with A103 2207WC.

Probably could be something related to this batch.

I don't know if other people with this problem has a similar batch.

This is from July of 2022. Given the tight supply situation then and now, this unit has been sitting somewhere.

Yep, get it replaced with a non-stained one.

There were lots of similar pictures posted here last year.

Get it replaced.

ok

Finally I have received a replacement and the new one is all clean.

Also with a newer manufacture date.