While this behaviour is a little unsettling, and possibly should be modified, I can see why it probably happened. Parity is built walking the disk sectors in order. Since the disk you pulled was only a 500GB, and you were building to 6TB of parity, the disk was already not being read because the parity build process was beyond the 500GB mark. Its contents (and all the other drives to that point) were already written to parity and being emulated by the rest of the drives.
I suspect if you had read file contents or written to that drive, it would have immediately failed and warned you, but since all you did was browse the TOC that was most likely in RAM, no activity was actually asked of the disk, so unraid didn't know it was missing.
Since one of the main selling points of unraid is the ability to keep some drives spun down if they are not needed, I think some effort has been put into not actively poking drives just to be sure they are there.
Unraid only fails a drive when a write to it errors out, so it's quite conceivable that a failed drive could hang out in the array for some time without being noticed. Regularly scheduled parity checks (typically monthly) are a way to be sure little used drives are still capable of participating in a rebuild should they be called into action.