The little icon/symbol within the triangle appears to be the symbol BMW uses to indicate coolant temperature. Then it also looks like there's something molded into the reservoir that says KALT/COLD with a drawing that would indicate an indicator stick's height (when the cap is screwed off).
If all the above is true...then I would say it is a coolant expansion reservoir/tank....especially if the engine has cooled down enough that if you were to unscrew the lid (no hissing as pressure escapes)...and a little float (hopefully) pops up.
For example, the pic below is from either my e38 or e39 owners manual...you can see the similar drawing molded into the reservoir tank (KALT/COLD):
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