After power supply is turned off, voltage still exists and is slowly dropping over a minute + long period
It is a normal phenomenon.

The reason it happens is because there is a capacitor which can hold a charge. Even when the power is off, it can keep it, or release it (which it does).

Sometimes you see it at a laptop connection that has in the power cable a small (ring type) LED, that glows when attached to mains, and when disconnected it slowly decreases in brightness until off; this can take also tens of seconds.

I don't know if there is an official term for this, but the releasing is called 'capacitor discharge'.