Jim - with that small of a volume of loss, I would place bets that most of it is simply your tubing
Most silicon materials
can be considered as porous (it isn't really porosity per say, but permeability). This is why 'common' silicon tubing gets discolored from additives. It is also one of the big reasons that we don't use silicon products at work for sealing electronics from moisture and contaminants (along with issues of out-gassing).
Silicon materials are actually also used as permeable membranes because its makeup. It allows diffusion to occur as a result of it's physical structure at the molecular level.
But 1 teaspoon every other week isn't a biggy