@phil_wong You would probably want to check the source- threat list activity for the duration that had unknown and find out which event is causing this? its most likely sending 'null' and causing this. If you can fix that, that could avoid 'unknown's coming in the rule/correlation searches.
@lakshman239 There's is no "unknown" source from my intelligence. Indeed, some of my incidents got a IP hit my intelligence. So the score of the exact IP keeps raising which is reasonable. However, the same incident also has "unknown" src or user, make "unknown" got high score as well.