Stuxnet was discovered by Belarusian antivirus expert Sergey Ulasen, who was working for Belarusian antivirus company VirusBlokAda. Later, he moved to Kaspersky. This is how it was discovered:

"It all started when the technical support guys informed me about a rather unusual case: a customer in Iran reported arbitrary BSODs and computer reboots. They forwarded me the help-request together with info about their preliminary scanning reports."

Read the amazing interview here: https://eugene.kaspersky.com/2011/11/02/the-man-who-found-st...