Nope. This didn't work. The internal resistance of the capacitor is too high to deliver enough current to prevent a power interruption. You get a very low current output for a long time, not a high current output for a short time. I found only very large supercaps such as car audio "stiffening" caps have low enough resistances, and those are about the size of a soda can so it's a no go.
@jediknight36 Replace the power supply. I've had a $5 chinese power supply for my pi2 give me the lightning bolt like 1-2years later. Opening it up, they used very thin wires that just gave out from the thinness and poor solder job. Replaced them with thicker ones and all's good. Zero issues with my official pi power supplies.
What could have caused it to appear momentarily like that, just as I was shutting down?
I wonder if your heater or AC was kicking on at that exact time or maybe a power brownout or if you're using an old surge protector, maybe it time to replace it. Those things don't last forever and can wear out over time from multiple power surges. Also if you're going to replace your surge, you might as well spend a little more and get a Battery Backup which will help with surges and brownouts. I've got this one that I plug my modem/router into and my main Raspberry Pi.
And is it something I should be worried about?
If it only ever happens once, I wouldn't worry about. If it keeps happening, then it could be an issue. Maybe a short in the power cable? Bad surge protector? Dirty power?
It would be interesting to know if anyone else had tried an alternative power source to their Pi when they've had issues with ghosting, because in all the threads I have read, I don't remember anyone else bringing it up.
@Lurker thank you for this detail, also super helpful for me. I am considering doing the same thing but using this switch (Pilot Automotive PL-SW26 Performance Toggle Switch with Red Safety Cover https://www.amazon.com/dp/B000GTMUUI/ref=cm_sw_r_cp_api_i_6uZQDbMXHJDGD). I don’t have any electrical experience - can I ask you what the difference would be here given this only has 2 connectors vs the 3 in the other switch?
You need to disable/remove the +5v/VCC power. This will then turn that USB cable into a data only cable. As your hub is a powered USB hub this won't matter. Thats why when the PI is off its still powered as the hub is now feeding +5v back into the PI. Not a great idea as its not really designed to be powered from a USB connector plus the problems this could cause with too much current draw through the USB connector that the PI needs
Either by removing the pin that supplies +5v in the USB plug to the PI or carefully peel back the cable insulation and snip the red wire for the +5v