Putting a hopeful spin on it, is there any possibility this is for example the location of a depot or something of the ground handlers in Stockholm and it is either in their lost and found office or making its way there?

I guess not, but like you I am surprised someone would actually steal these from an aircraft given they are traceable.

The other vague possibility is it was actually a passenger who took them to reunite them with the rightful owner, which may be easier than handing them in at an airport. That's what I'd do with a wallet for example. But headphones, probably not... Oh well.