Security guards are not the only ones, of course, who do not show their whole face (especially their eyebrows) to look more intimidating/dangerous. Warriors in Africa have shaved their eyebrows for centuries to look fierce. Members of the U.S. Marine Corps have shaved their eyebrows and hair except in the middle of their head (a ‘Mohican’ American Indian origin) to look dangerous.
These security guards do this for the same reason, and I am sure there are other groups that do this for the same effect. Half of providing security is to make yourself look like someone others do not want to deal with in a negative manner.