A head of security was a person in charge of security.
History[]
While spying on Snowcap University for Ophion Oil, Paul South worked as head of security. (COMIC: Blood and Ice [+]Jacqueline Rayner, DWM Comics (Panini Comics, 2015).)
Former British Army major Norman Baker worked as head of security at the Wenley Moor nuclear research facility in the 1970s, after a bad — but unidentified — slip-up led to his having to resign from the Army. (TV: Doctor Who and the Silurians [+]Malcolm Hulke, Doctor Who season 7 (BBC1, 1970).)
An unnamed head of security apprehended Adric, Nyssa and Tegan after finding them and the newly regenerated Fifth Doctor in the grounds of the Pharos Project at Cambridgeshire in 1981, which were secure premises. (TV: Castrovalva [+]Christopher H. Bidmead, Doctor Who season 19 (BBC1, 1982).)