Thanks for that.
Right, there are three possibilities.
One: It's via the air holes. It could be a miniscule amount that's collecting under the AFC ring and flowing down to the base.
Two: Leaking through the bottom fill hole. Is the tiny o-ring still there? If not, you'll need to get some 1mm (cross section) x 2mm (Inner Diameter) o-rings.
Three: Via the 510/ positive pin - especially if you've adjusted it at any time (or if the air holes were drilled too deeply and have punctured the 510 shaft (as has happened with one of mine)