Hello,
I have RaspberryPi4 + USB C power adapter (5.1V/3A) connected to smart socket (Z-wave).
My problem is, that even when smart socket is OFF, red power led on my Pi 4 blinks every maybe 15-20 second. I thought it could be failing socket, but other socket from different manufacter is doing the same.
What can be the issue here?
Thank you.
Your printer is likely backpowering the Pi - or some other device. The power will be coming from somewhere. Details and a link to the fix below.
So I unpluged my Prusa Mini from Pi and blinking stoped! That interesting because printer is in another smart switch which is OFF as well. And shouldn't be printer backpowered by Pi and not Pi backpowered by printer? 
Electricity flows both ways, agreed it seems odd that if everything is off... Maybe there is something still powering in the setup somewhere.
When I unpluged USB C power from Pi, blinking stops... That is strange because backpowering should work regardless, shouldn't it? There is only Raspberry Camera connected to Pi via CSI and small 2-pin fan powered by Pi's GPIO.
I pluged printer directly to mains (to remove that other smart socket variable) but it still blinks, so that socket is not as issue.
Does it still blink when you remove the fan?
Yes, it still blinks even when fan is disconnectd from RPi.
Oh sry I didn't read the OP properly. I thought this was about underpowering.
It's possible that it gets the +5V from the power adapter and the GND from the printer.