My guess is it wasn't labeled an inverted delta part meaning it's not critical and thus not going to stop the line if it's not installed. Someone failed to cross that "t" when they were mapping the job layouts. I guarantee that has been fixed since they discovered the fault.
If it's a new part all together that wasn't part of the old model, or on a different station/line, and the line operator and/or hi-lo driver weren't instructed that the part was supposed to be there, I can see it happening.
I'm guessing they noticed they weren't there when either a) the part just suddenly appeared on the line one day (plausible) or b) someone did inventory in the material handling department and noticed there was a massive surplus of this part and alerted management (more likely)