When a device is selected in Device Manager, the leftmost button on the toolbar is the "Properties" button and the one immediately to the right of it is "Scan for hardware changes". When the topmost item in the list (representing the computer itself, not a particular device within/attached to it) is selected, the "Properties" button is hidden, thus the "Scan for hardware changes" button moves over to be the leftmost button, however in this button configuration, when hovering over the button to scan for hardware changes, it brings up a tooltip which says "Properties". It should say "Scan for hardware changes".
Also, when only "Scan for hardware changes" is showing, hovering over the space next to it (where the "Scan for hardware changes" button would be if it were in the other button configuration) shows a tooltip for whatever was just to the right of "Scan for Hardware Changes" prior to changing what item was selected (in the case of the attached image, it's "Enable"), even though there is no button there.
Note: This only appears after the toolbar button configuration has changed. (You won't see it when you first open Device Manager, you have to select a device, then change the selection back to the computer.)