Whenever a new wine has been put into a barrel with a solid stopper, I get some seepage, as well as when I top up my barrels. I've decided that since my barrels get filled / topped to the underside of the bung opening, that when I force the bung in, pressure is created in the barrel and since there is no air to escape, the wine seeps out around the bung. The same is experienced when a recently filled / topped barrel was filled during high atmospheric pressure, and the pressure drops.
I've switched to the vented silicone bungs, now the wine seeps out of the vent on top, which is absorbed by a paper towel wrapped around the bung. Once a little evaporation has taken place in the barrel and you have air space in there, it doesn't happen.
I frequently spritz a little KMS solution on and around the bung to keep stuff from growing on the overflow, it also completely removes the stains from the wine.