Goes away by magic??
One plugs the lenspen back into its cap and gives it a half turn. The instructions say that "replenishes" or "recharges" the cleaning surface?
http://www.lenspen.com/media/Core%20...20-%206.21.swf
It works best on fingerprints, skin oils etc! I haven't experimented with much else.
I note FAQ includes:-
Q. Why do LensPen and MiniPro leave a residue on my lenses after I clean them?
A. Excess cleaning compound might be left on the lens after cleaning. If this occurs, pat the tip on a clean chamois or a clean soft cloth to remove the excess compound from the cleaning tip. Pressurized air or a "blower brush" will also work to blow off any excess compound on the lens.