Have you unplug the power cord and see if the light turn off, after shutdown could be a PSU issue, do you have another PSU you can test and see if you get the same error.
Normal situation, the residual currents of the capacitors are spent on the operation of the LED. In other cases, you don’t visually see it, but it happens. For example, ventilators slow down milliseconds longer. I don't have references to technical explanations to describe in more detail. For this reason, it is not recommended to turn on the equipment immediately after turning it off. Residual electrical voltage with the addition of a new one may react adversely to components, including damage.
Yes, flipping off the switch on the back of the power supply makes it go off. The residual power explains the "fading" off rather than just "blinking" out, but it should not "fade" back on a few seconds later. I looked over there and thought I left the darned thing on all night, but it was actually off but with a fully lit power light/LED. I guess I'll have to rip it all back apart and test with bare minimum plugs and hardware, one at a time. One other thing to note: If I kill the power supply, wait for the LED to go out, then flip the power switch (on the power supply, not the PC case) back on, it stays off (the PC case LED) until I actually power on the PC and shut it back down.
Now it sounds weirder than after the first reading. Disconnecting all external peripherals and checking is a good idea. Finding the cause "inside" sounds difficult if the symptoms are observed only with the motherboard and power supply. I can't give advice here.
sounds like after you power off the mobo retains electricity......try #5 suggested and see if the issue continues.
does older cases in the front use a few single lead connector for power and reset make sure does connectors are connected properly you mite have them in the wrong and is cause of this error.
The most obvious reason is just the led plugged on the wrong pin. Pretty common, given there is always the 2pins/3pins choice, where sometimes 3pins connector on MB has just a duplicated negative or positive, but in some more modern mobos the three pins are meant for 3 pins leds (with standby color change or alike). Other theories, are just theories. Occam's razor always works.
Yeah, it's an ancient Antec case (heavy as F!) and the color coding of the case leads appear totally random with no polarity marking, but it was in the right place/orientation, and it is the old 3-pin with a blank middle lug, and plugs into a separate area near the PC Speaker. First try I had the HDD LED reversed and had to fix that. Edit: Seems to be fixed. With my 15 year old Seagate Baracuda 320GB HDD removed, and replaced by a M.2 SATA MX500 SSD, the problem is gone...for two days. I did not see any obvious problem with the drive and it still worked fine, but who knows with 15 year old electronics under it. Edit after a week: Yup, it was the HDD. That or some random thing I can't fathom that "fixed itself" in the process of troubleshooting.