Oily substance on top of motor?

[letsgetweird]

I went out to work on the bike (master cylinder) and found a oily substance on top of the motor but I have zero idea where it's coming/came from? Valve cover bolt leaking was suggested by a friend but thought maybe someone might have experienced this before?

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[Darkstar]

show us a pic dude

[J_Walker]

valve cover bolts just have an O-ring under them, I don't know the exact size, but they are just basic o-rings.

Could be the breather vent too, on the VERY top of the engine, is the hose still connected? between the very top of the engine, to the airbox?