Sennheiser Service response to my question to them about the small silver sticker inside the left ear cup, which can be seen glinting (annoyingly to me) behind the grille:
"It sounds to me like that is one of our new holographic product ID tags which we began using recently to help customers identify real Sennheiser products opposed to counterfeit products. There should be no harm in removing the sticker if it bothers you, but just stick it inside your headphone case and keep it rather than throwing it away. You might end up wanting it in the future to prove the authenticity of your headphones".
That holographic sticker is on (the left cup of) my very recently acquired HD600 (which I bought from an authorized Senn dealer, RazorDogAudio), as well. I don't think there's really ever been an issue with HD600 fakes.. but I'm glad Senn's including something like this as it does offer peace of mind.. especially if someone plans on reselling their HD6x0.