Where does this part come from?

[Frenchfries]

Guys, While cleaning up and preping the bike for a 2,000 miles ride starting tomorrow, I found a loose part.
It is a rubber cap transparent of rather large dimension. I found it between the hydraulic clutch command and the shifter on the left side of the bike.
Any hint on where it goes back and what it covers would be greatly appreciated.
Thanks

 

[Dogdaze]

Look inside the airbox....... it's a cap for a breather hose/ drain cap

 

[Frenchfries]

MERCI, Ami Helvète...
See below the part properly installed at the bottom of the air box (gas tank lifted).

 

[patrickg450]

BMW owner prophylactic?

 

[cb0802]

Its an oil separator. The top of the engine has a hose that allows oil to escape in the event of over pressure. The hose runs into the bottom of the air box. The separator then collects the oil for easy removal and cleaning. The alternative is to have it collect in the airbox and potentially enter the intake.
If you ever overfill your oil you'll see what I mean. And how I know.