MacOS – Mac is using 100% CPU while it is OFF

Try opening a terminal and type

  ps -AaC

The CPU time there is cumulated time spent executing, but maybe a process will show with an all time cpu use that sticks out somehow.

There also seems to be an answer to a similar question here which suggests Intel Turbo Boost may be the culprit.

You have classic symptoms that thermal issues are causing kernel task to preemptively use up CPU to prevent overheating. If you haven't reset the SMC, try that one time. If the System Management Controller is actually stuck, the response to changing thermal measurements might lag or leave blowers on higher than they should be.

• kernel_task hundreds of % CPU yet cpu frequency is being lowered
• CPU usage is too high with Yosemite and an external monitor
• kernel_task consumes 500% CPU on AC power, sometimes
• MBP early '11 kernel_task over 300% when unplugged
• MacBookAir kernel_task at 300% and rising

The answers above all point to things you can do to measure swap, processes, etc... but focus on thermal sensors (hardware issue) or external temperature (environment) which you can cool artificially to remove the slow down that's designed to manage heat.

If you can't measure the case temperature accurately with an infrared thermometer, try software like https://bjango.com/mac/istatmenus/ to check on the individual temp sensors and correlate them with the "spikes in CPU usage" by kernel_task. Also, resetting the SMC one time might be worth your while. If it's stuck, the response to changing thermal measurements might lag or leave blowers on higher than they should be.