Small Engines (Lawn Mowers, etc.): kubota diesel, kubota diesel engine, rubber plug

Question
I have a kubota zero turn with a 28 hp Kubota diesel engine model D1105.   It has 450 hours and runs great --but the other day I noticed oil leaking and found a round rubber plug in the area of where the feul mixes (it is on the side of the engine) blew off.  I lost about half of the engine oil before I noticed the problem.  What would cause this and what do I do to make sure it does not happen again?  Do I just stick the plug back in and not worry about it?

Answer
Rob
The rubber plug is a blow by pressure relief. It only lets oil out if the pressure is to high,the problem is sometimes they just get loose and they wear out Refill the oil and keep a eye on this if it happens again get a new plug or bring it in for service to a good repair shop that services that equipment.