I too have seen that every time I pull the silencer off my carbs and others. I think it is from unburned fuel hitting the front for the silencer and sitting. My guess the gas evaporates away, and the oil is left. I am certainly no expert, but I haven't had any problems, and like I said, have seen it every time I get down to the gasket. My experience is that it usually is just a drop or two. If the puddle is any size, that may be a sign of some carb issue. Someone will chime in who can speak with authority.
SM