I think the hope to remove that debug message alone, without modifying source code, is slight.

But I can explain that magic number, 563, showed on the screen. That's a window message id. It is defined as a constant in winuser.h, identifier WM_DROPFILES, value 0x233, which is 563.

It is a windows message being sent to the receiver of the drop action. If you debug your code into a little bit more detail, you will see that the message bothered you shows up, right before entering your dropCmd sub. The sequence makes sense, as once the receiver got notified by receiving that message 563, it then calls your dropCmd sub.