I was under the hood the other day and noticed that the cap to the brake fluid reservoir is MIA!

I have no clue when this disappeared, but I'm ready to blame the service guys because I have no better explanation for it...

Any idea if this is a simple item that the parts dept would carry? Should I just walk in and hope that they have one in stock? Is this a standard/interchangeable part that I can pick up from any dealership or is it some special German made w208-only thing (hope not, but you never know).

Thanks