Get it replaced ..And in the mean time fit a piece of plastic bag over it wit a rubber band , Might need a small hole in the plastic i dont know . I recon your service guy left it off after he checked the fluid in there .
Those caps are generic. Virtually any European car will have the same cap.
If the cap has been missing for a while you will need to flush the fluid. It is ordinary dot 3 or dot 4 brake fluid. As with the brakes, the system should be flushed every two years. That is true whether you use it or not.
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