My PC was unplugged for 3 weeks. Came back, plugged it into outlet and turned it on, then came a mighty BANG and smoke. The PSU is (was) a 5.5 years old Antec TRUEPOWER II TPII-380 380W. Would this have happened if I had left the PC plugged in outlet?
Components are always stressed at power on/off times because of current surge. Given the age of your PSU I'd be inclined to blame a capacitor but it could have been anything. Leaving it turned on would, most likely, have extended it's life but you have to trade off the running hours against the risk of shortening component life. In general, because I use a ups I leave old equipment turned on or in standby but turn new equipment off.