What’s that on my Arc ultra?

It looks like a small creep of the adhesive used to attach the end plate and the adhesive is now collecting dust. The dust collection is likely accelerated by your attempts to keep the unit dust free.

Attempts to remove the adhesive will likely create damage over a larger area.

Examine a less visible area for similar creep and experiment with techniques to remove the film. I’d be tempted to use the edge of a soft business card to buff the area. A mild solvent, such as water or alcohol may help, however, the buffing and solvent may damage the overall sheen — resulting in a larger, more visible defect. If you can perfect your technique in a not so visible area, allow a few weeks for the invisible area to “cure”, then reevaluate the effectiveness of your technique before applying the technique to the more visible area. I suggest the wait because your technique may remove an overcoating that will expose the underlying area to attack by the atmosphere.

The second view shows that the junction is collecting dust — likely pushed into the junction while you are dusting the unit. You may be able to blow the dust out of the junction if this area is not “sticky”. A very risky approach would be to dye the collected dust black. Be sure to practice your technique in a less visible area.

​@buzz thanks for chiming in. I  really don’t know what it is. it is not sticky, it does not protrude, it can not be picked by the fingernail, nothing. And not that anything is missing, rather there is too much material. If if is indeed hardened glue that sipped through i think I can live with that. My theory it is plastic that appeared when it was poured in a mould of the speaker enclosure, it that makes sense. And also, i haven’t noticed any other defects anywhere else on the Arc. I will take a photo of the opposite end. I am sure it will not look perfect as it does to the naked eye while it looks pristine in real life. I am telling this so I can further ellaborate how barely noticable the defect i have showed you is. But it does bother me and it is a quality control issue, and that is something unheard  till now from Sonos. And the app was perfect yesterday. I don’t know what are your experiences?
Neven

I suppose that the problem could be the inverse of our assumptions and a final coating is missing, rather than something extra.

With respect to the App, it’s not yet complete. Depending on how a person uses their system, it might be “perfect” or “disaster”, depending on the features that the person expects or wants to use.

On my white arc ultra, rear left side as you look at front of unit mounted on the wall - today I noticed a piece of clear plastic wrap that needed to be peeled off the left side, the more square piece at the back (which touches the wall but faces off to the left). It was about 7mm x 40mm. I didn’t notice any other plastic wrap that needed to be peeled off during unboxing a few months ago? Is a small piece of plastic wrap to be peeled off on the non-perforated plastic part of the arc ultra normal - and is this by any chance your issue too? Some plastic film/tape sticking through?

No such thing. I can not feel it under my finger nor grasp it in any way. I think it is a small part (residue) of plastic thet went outside of thr mold. 
thanks for replying. I will contact sonos directly. If there is a quality control issue that has been registered before they will know about it. I will accept roam 2 as an excuse :-)

Neven

Hmm strange. Good luck getting a resolution. I must say, although the packaging may seem nice, certainly for the arc ultra and era 300´s, it is very easy to carefully remove the lock tabs and internal bag sticker, potentially use the product, then reseal everything and return it - it would not be evident that the unit had actually been opened and/or used, and would likely be re-sold as new. I much prefer the tearable tab on the sub 4 for example, which is clearly irreversible. At least this way you can be sure any such issues are from production.

Out of interest - did anyone else find a small piece of clear plastic wrap/tape on one side of the arc ultra only? I assume it was to protect the back piece during assembly and should have been removed prior to the item being ultimately boxed and shipped to be sold? The plastic had dot perforations along the edge as if it is supposed to be ripped off at some point.