ACC power doesn't cut right away
 

I'm trying to troubleshoot or determine if/why my 12V ACC power doesn't drop when the key is removed. Using a multimeter, I measured voltage across the 12V ACC pin of my stereo's harness to a chassis ground and watched as voltage slowly trickled down (about .01V / second)

Anyone has any experience like that?

as a guess Id say that as its the ACC circuit that the voltage you are seeing is being back fed from something like the HU, as this contains caps that would need too discharge.

What is a HU?

Head unit (stereo)

No head unit at all.

All stock accessories.
No turbo timer, no gauges etc..

is it on the same circuit as the interior light ? as some cars have slow dimming after the key has been removed to allow you to see what you are doing as you lock the car up, ie left wallet on seat

It is very weird problem.
I measuring the voltage from Ignition Switch near the keyholder. It is the same!
I thought it is the ignition switch. I opened the inside of that switch and there was some loose contacts. It is copper and there were oxidations inside that switch.
It didn't helped either.


I'm thinking if a relay can hold some amount of electricity inside the coil?

I checked all the fuses and relays.

But I have read there are some more relays behind cluster panel. I'll check that too