Hopefully it's an oil leak and not coolant. A slight oil leak is easier to live with than a coolant leak in the motor. The dealer you took it to specifically said "transmission" so hopefully that's where the leak is. When you say the "leak around the plastic cover" is it coming from the round cover at the end of the motor or is it further back near the chain sprocket? The motor itself has no oil in it to leak, only coolant. Behind the motor there are two places that could leak transmission oil--the output shaft seal where the front sprocket is attached and the gear shift shaft just below it. In addition, there could be oil coming from the transmission breather but it would be noticeable as it would get over a lot of parts. You could also be leaking brake fluid from the clutch slave cylinder which is mounted directly in front of the forward chain sprocket. If it is leaking enough to leave an oil spot on the floor while sitting then what you want to do is clean the entire transmission with a degreaser, dry it off and park the bike until you see a drop of oil or coolant. Then follow a line straight up from the floor to the area on the bike where the leak is coming from. Because the bike sits on a side stand and leans to the left, the leak could be on the right side and just flowing to the left before it drips off. So look for any traces of oil that might have run under the motor/transmission. If it isn't coolant (and it's a slow leak) I would just keep an eye on it. If it is coolant it's possible the leak is at the coolant hose connections to the motor. However, there is a thread about coolant leaks it the motor itself which would be the worst situation to have.
