I left some details out, the capacitor was removed from the electrical circuit, charged with a battery, left for five minutes and had no voltage. Then I left the voltmeter connected, charged the capacitor again and when I disconnected the battery, I got to watch the voltage rapidly go away. A perfect capacitor, out of circuit, would hold the charge forever.
In the real world a new capacitor should hold the charge for hours. Less than five minutes, as described in the manual, means the capacitor is almost/already dead and needs to be replaced.