The 'Show / hide properties' button is located above the project-tree. Do not confuse it with the 'fit to screen' button . The properties of what is highlighted in the tree (Attachment-, Note- or Sheet properties), is shown in the panel on the right, if the 'Show / hide properties' button is clicked. This is a toggle-switch, so if you click it again, it disappears. Notice the difference from the 'Fit to Screen' button. Then what is behind it may be shown, such as an opened Sheet. This is an example of a Sheet window that is partially hidden by the panel:
Another way to either bring the Sheet panel or the Sheet window to the front, is to click a part of it that is still visible. For the Sheet panel, this is also true for the project tree. Speed keys for the Show/hide properties button are [ctrl]+[s].