For anyone else with Microserver, it was the molex to Sata convertor for the HDD in the top ODD bay. Nothing on the back plane.
It is unlikely they'll accept it back unless the marks come off. Run a full battery of tests on it, if it passes them all then its likely fine.
Obviously would pay to backup your data regardless and inspect the connectors the two drives were plugged into. Maybe switch to some others on the PSU to be on the safe side.
Maybe the spider, which made the cobweb, crawled over some live contacts and fried