A 400 V 120 uF ElCo was in a device circuit so both were used and the Elco got charged then the device was unplugged and the Elco was shorted in 3 sec or so (very first is big spark) and DMM read it's 25 V

next nearly an hour it's read, it's 121 V

then it was shorted in 3 sec or so and DMM read is 4 V or so

next nearly quarter hour it's read as 13 V or so

then it was shorted in 3 sec or so and DMM read is 2.9 V or so

next 9 s or so the DMM read is 9 V or so

then it was shorted in 3 sec or so and DMM read is 0.5 V or so

next 9 s or so it was shorted in 3 sec or so and DMM read it's 0.03 V or so

next 9 s or so it was read and it's 0.21 or so

The last 5 step is when it's been separated from circuit, ensure it behaves on its own

Pretty scary voltage rise, so help answer
Is this Capacitor / Elco behavior acceptable & normal or no, either, what actually scientific explanation on this Elco having a voltage regain/rise behavior ?

 

What is the complete device, it sound as though it is not just a stand alone capacitor device?

 

It is called dielectric absorption.

Even though you neutralized the charges on the plates of the capacitor when you short the capacitor the dielectric still retains some degree of polarization. Over time charges will build up on the plates.

Reference: https://en.wikipedia.org/wiki/Dielectric_absorption