The label "Show uknown events" is imprecise, because when it's unset, we also don't display insignificant PROBLEM and OK events.
It's really confusing. I opened the events page, saw a whole bunch of PROBLEM and OK events and a few UKNOWN events. I've unchecked the checkbox and all of the events have disappeared. It took a while to figure out, that all of these events where just insignificant. I think it would be better to rename the checkbox to something like "Show insignificant events".