Still probably from the bypass. Water can retain heat for a long time in a large tank. Is you systems pressure switch driven? Are you using total shutoff guns? If so, you should use a true unloader if you only have a pressure regulator. That will reduce energy from the motor and actually 'unload' the head when the gun is released, which will save lots of electricity and wear on the pump and motor. Lastly, are you leaving the system 'on' overnight where the pump will turn on and off cycling because the pressure on the prep gun leaked down?