It's not just those areas where it gets in - it's also the tiny gap that some 920's have between the screen and the case. You could see fine dust in that gap on mine which was removed/moved by blowing on it.
Two things come to mind:
1. Do you lock your phone with a passcode? If so then it won't appear on your PC when plugged in until you type in your passcode on the phone itself.
2. Windows failing to assign a drive letter to the phone - go to the Disk Management area of Windows and see if there's...
Clearly you've never listened to Pink Floyd's Dark Side Of The Moon on vinyl then, or many, many, many other classic albums whose tracks flow into each other rather than having a distinct gap between them.
Yeah, this is the main problem. Getting a call from Nokia today to arrange collection of my 920 - they refuse to do a straight swap for a new one until this one has been back to them three times which, unless they can deal with the design flaw in the product, is going to happen by April/May...
Mine's given up the ghost completely now after two month's ownership. I've had the problem for a few weeks now and blowing into the earpiece did help for a while, but not any more. The only way I can hang up calls now is by double tapping the power button and very quickly tapping End Call...
You returned your phone for that? I can get it to do a similar thing on mine if I wipe my finger over that area (therefore depositing some finger 'grease') but it can't be seen after I wipe it clean again.