A lot of us use marker pens on our glass to determine cut lines, indicate areas that need grozing, etc. These pens have a variety of names – felt tips, Sharpies, paint pens, fibre tips, permanent markers, laundry markers, and many other generic and trade names.
Most, except the paint markers, contain water or spirit based colours. Many of these pigments are reputed to burn away during the firing of the glass.
Paint markers and the ones that contain metallic colours rarely fire off. They are more likely to fire into the glass. Some people take advantage of this fact to quickly add marks that will survive the firing.
I no longer trust anything to burn off. Even if the marks do apparently burn away, the residues are sites for devitrification to begin.
I clean all my marks off before firing. It only takes the marks to be fired into a favourite piece to convert you to cleaning. If you use paint markers on black glass or coloured felt tip marks on clear, clean it all off before firing. This removes the chance that the pigment will remain throughout the firing and ensures the glass is spotless when it goes into the kiln.