Hardware Check Screen

I turned my computer on and the hardware check screen came on. So, I ran the short test, 1 error message (4SNS/1/4000 0001: IDOR-0.040). I then ran the longer test - and same error message. However, I am not concerned about the error message as it just means that my computer wasn't plugged into a power source when test was run. What I am concerned about, is why did this hardware check screen appear in the first place, as I did have the MacBook Air Hardware Test Flash Drive inserted. Nothing was connected to my computer at this time, and I was not in a wireless area.