Posted this some time ago and it was left unresolved:

"Why does this happen?
I hide all menus so that my sheet view goes all the way to the top of the screen, except for the Excel Application bar. Occasionally, after running code, the screen will cease to redraw the very first line on the screen, showing instead this grey bar of nothingness. And I have yet discovered a way to have Excel redraw this missing portion of my screen."

Posting it back today since I have a solution to the above.

I was getting this grey bar where the menus would normally have appeared. My program removes access to these menus on start-up. By trial and error, I figured out that it was appearing once we were moving between workbooks.

Discovered that my "grey bar" is the place holder for the custom menu bar which is triggered by the windows activate event. If you right click this grey bar, you can select menu buttons, even create new menus.

To prevent this grey bar from coming back. In the windows activate code for the open workbooks, you simply include a For Loop, that will enable and then disable all command bars. For this to work, make sure you re-enable them before you disable them. Setting screen updating to false will hide this from the user.