THat's right, what '9 lives' said-(nice orange cat)
You probably have at least a hairline crack on that inner cover or plug, maybe on the outside one too . If you want to be really lazy, and economical, you could just fill the crack with something, with what depends on how clean you can get it. If you can get it really clean, fill it with superglue, and let it go off really good. That'll make it stronger, too, and maybe it'll last till you get rid of the bike. If not so good and clean, try some gasket material, silicone or bondo, applied to the inside of the part.