What a real hero! Is it known why he suspected it wasn’t real?

Edit: I see it here.

>In explaining the factors leading to his decision, Petrov cited his belief and training that any U.S. first strike would be massive, so five missiles seemed an illogical start.[11] In addition, the launch detection system was new and in his view not yet wholly trustworthy, while ground radar had failed to pick up corroborative evidence even after several minutes of the false alarm.