To reproduce:

First you need to have two displays, the resolution does not matter. Adjust the displays so that the second is higher that the first one. You can do this in the Display settings window of your Windows (Right Click -> Screen Resolution on desktop on Windows 8).

Add a RadDropDownList to a form. Move to form to the top of the second display. Open the DropDown and you will see that its location is not correct.

The issue also occur with RadMenu.

Workaround:

this.DropDownList.PopupOpening += DropDownList_PopupOpening;
this.DropDownList.PopupOpened += DropDownList_PopupOpened;
this.DropDownList.PopupClosed +=DropDownList_PopupClosed;
void DropDownList_PopupClosed(object sender, RadPopupClosedEventArgs args)
{
    RadDropDownListElement element = sender as RadDropDownListElement;
    element.PopupForm.LocationChanged -= PopupForm_LocationChanged;
}


void DropDownList_PopupOpened(object sender, EventArgs e)
{
    RadDropDownListElement element = sender as RadDropDownListElement;
    element.PopupForm.Location = savedLocation;
    element.PopupForm.LocationChanged += PopupForm_LocationChanged;
}


void PopupForm_LocationChanged(object sender, EventArgs e)
{
    ((Control)sender).Location = savedLocation;
}


Point savedLocation;
void DropDownList_PopupOpening(object sender, CancelEventArgs e)
{
    RadPopupOpeningEventArgs args = e as RadPopupOpeningEventArgs;
    savedLocation = args.CustomLocation;
}