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This problem is very common, particularly with Epson scanners. I've had new
scanners from Epson delivered like this. The problem is condensation forming
under the glass. It won't disappear over time, it will get worse. It is made
worse if the scanner is left in a moist atmosphere The effect is the same as
on a bathroom window. If you leave a windowpane uncleaned in a bathroom it
will start to look exactly the same as your scanner over a period of time.
It's the tiny particles of dust/dirt/grease etc that stick the surface when
the steam is formed and condenses on the glass. It's the same with a
scanner; particles are left on the glass. It is very difficult to remove
without taking the scanner apart and cleaning the glass. I doubt that using
silica gel will remove it. But please do let us know it works.

Every Epson scanner I've owned has developed this problem over time. Someone
once suggested that with Epson its the plasticizers in the plastic that gass
off. This may explain why Epsons seem to be particulary affected
..Fortunately, someone posted disassembly instructions for Epson scanners on
one of the scanner newsgroups a few years ago. I suggest you actually post
to one of the scanner groups in case someone knows how to take a Canon
apart.

pdm