It has been three weeks since I encountered the issue. I have now powered down and double checked the sata connections on all drives and I am running a new parity check. If this also returns errors I will attach the diagnostics.
To answer your questions:
The first check was scheduled but the second I ran manually the next day after seeing the errors. I have it scheduled once every three months.
I will change the scheduled check to be non correcting. Thanks for the advice.
The "lots of power cycling" was because I was migrating data from a hardware RAID5 with an external raid controller. I won't go into the details but it was a nightmare. I'm glad its over and the data is now on the unraid array.
I am shutting down using the webUI