Thanks for the input, now I'll tell you what I found.
Arrive on site and heater breaker was not tripped. Check on all connection points, with power off metered terminals to ground found no continuity, found some non relevant loose wire nut connections (corrected). After checking for other issues directly with heaters and finding none HO indicated again that it sparked when she turned off heater and heard popping. She indicated she had it turned up high to take chill off. The only noise production was from thermal expansion of metal parts, and found some buildup of animal fir on fins. Made some adjustment to brackets and cover. Further interviewed HO on specific occurence turns out heater breaker was not breaker tripped it was another single pole that was tripped. Upon pressing for more details it turn out the light that was plugged in next to the heater had gone out at the same time as the sparking.
She has a light that had no receptacle near it. Found out she had plugged the light into an extension cord that ran across the heater, checked cord and it had melted through shorted out onto heater making sparks and eventually tripping the SP 15A.
Repaired cord and advised that she not run cord over or immediately adjacent to the heater.
One more time a HO leaves out important details and wants to find a fault with equipment and not tell you about the stupid thing they did that contributed to what happened. This time no one injured and no property damage, other than the poor light cord.