Perhaps this post should be under JR's Entropy thread...Dielectric absorption is the name given to the effect by which a capacitor, that has been charged for a long time, discharges only incompletely when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage from time-delayed dipole discharging, a phenomenon that is also called dielectric relaxation, "soakage", or "battery action." https://en.wikipedia.org/wiki/Dielectric_absorption.
Recently, while checking a 48V switching power supply, I charged a 47µF/63V Panasonic cap to 48V in order to bypass the supply output.
It needed a simple RC filter to be useful.
Before putting the cap away I discharged it for a few seconds into a dead short.
Being curious I measured it a few seconds later and noticed it recharging.
This is due to dielectric absorption...something I've had to deal with before in mic preamp phantom coupling caps.
It was charged and "discharged" about a week ago.
I checked 2-3 days ago and it was at about 1.5V
Today its still at 1.5V with no sign of discharging.
I'll hold onto the cap awhile longer and measure it. (Until I get bored with it.)
I think we'll find it will hold the 1.5V for a very,very long time.