Mainly looking for during background check as simple
There are several different checks. Criminal check deals with verifying if person has any criminal record and it is basically covered by police certificates. Background check mainly verifies education and experience credentials.
Then there is background/security check which is conducted completely outside of and independently from Canadian immigration by Canadian intelligence and security agencies, with and/or without cooperation of proper authorities in all countries applicant resided since age 18 (for some nationals since age 16). It looks into applicant's background, activities, associations, etc. to determine if person was and/or is involved in any espionage, terrorism, organized crime, radical groups, crimes against humanity, etc. - how it is done and what is found is classified and CIC only receives "positive" or "negative" decision.
When it is required for employment reference check, they used to do site visit or calling former or current bosses, or any people you list as a character reference, even they can check Facebook, Twitter, LinkedIn etc in the social media..
Simple enough?