70-200 f/4L IS date code UY with dust inside

I have a 70-200 f/4L IS with a date code UY, which I'm sure is 2010, which I have had since March 2010. I have recently noticed dust on the inside of this lens and wondered if this is normal since this lens is supposed to be resistant to dust and moisture. I do not have a filter fitted to it so I'm not sure if that one is required to make the dust sealing complete, the lens manual does not state that a filter is required. Can any one else with this lens please advise me if they have dust in there copy?

On a side note I do appreciated that dust resistant is not the same as dust proof and that dust inside a lens does not affect image quality, I'm just wondering what other users of this lens thoughts are?

Thanks

It's really only dust and moisture resistant with the rear cap on, or when it's mounted to a body. If there's dust in the body, it can transfer into the lens, or you can pick up dust when you have the rear cap off.

This lens does not need a filter to complete the weather seal.

I guess if you don't have a sealed body like the 450D/XSI then the lens is not really sealed at all?

Debris will always take the path of least resistance...........

take apart the front element and clean it