To reprodiuce:

public Form1()
{
    InitializeComponent();

    for (int i = 0; i < 50; i++)
    {
        this.radDropDownList1.Items.Add("Item" + i);
    }
    this.radDropDownList1.DefaultItemsCountInDropDown = 25;
    this.radDropDownList1.PopupClosing += radDropDownList1_PopupClosing;
}

private void radDropDownList1_PopupClosing(object sender, Telerik.WinControls.UI.RadPopupClosingEventArgs args)
{
    RadDropDownListElement ddle = sender as RadDropDownListElement;
    Point relativeMousePositionInPopup = ddle.Popup.ElementTree.Control.PointToClient(Control.MousePosition);
}


Workaround: access the RadDropDownListElement.PopupForm instead of the RadDropDownListElement.Popup.