The older diesels (before all the emissions requirements) had a tube that comes off the valve cover or crankcase (called a Slobber Tube) to allow "blow by" gasses to vent to atmosphere (go onto ground). That's where the name "Road Oilers" came from when talking about the old Detroit Diesel 2 stoke engines (as they had a lot of blow by).
Usually there will be a trace of fume and some signs of oil - in a normally running engine.
If the engine has a lot of blow by (i.e. worn piston rings) there can be a significant amount of blow by. See this example - go to the 8:30 minute mark at the end
In that case you have a problem. But for a few small drops and a little smoke, it is not an issue.