Anyone ever have to use the power switch in the back method like me? ( I guess the proper terminology for that switch I've just learned is the "rocker" switch )
It's actually the "master power switch" and what it does when set to off, besides the obvious, is to remove the ATX Form Factor Standard required
+5Vsb standby voltage from several points on the motherboard. A master power switch is not required by the standard, however - so not all PSUs have them. For those PSUs without a master power switch, the +5Vsb voltage is applied across several points on the motherboard when the PSU is plugged into the wall. This is why is it always best to unplug from the wall whenever you do maintenance - even where there is a switch, since they are a $.05 part that could fail.
Note the function of the computer's front power switch is configured by settings in the BIOS. The settings allow for instant off, press and hold for 4 seconds to turn off, and depending on the BIOS, may have other settings. If me, I would probably replace the motherboard battery too - just because the board is 4 years old and those batteries are cheap.