Battery leakage or something

On the bottom of my black macbook, on the battery lock knob, I always see some sort of substance and when I wipe it off, it always reappears later. It looks just like what finger grease looks like. If it matters, it's right in between the lock and unlock symbols.

OK what ever it is, it isn't good. I think you need to talk in to Apple store for them to have a look. What colour is the liquid and does it do it when the battery is out of the machine?

Take the batt out and wrap it in a papertowel overnight, nex morning, check for leaking