He was also not present before the JFKA. He left around 12 noon. And presumably told his work colleagues he was leaving.
In this sense Givens was not missing. Much like Frankie Kaiser was not missing. They knew where they were, but not Oswald.
"Missing" and "not present" can mean two different things.
It could be said that after the assassination, all the warehouse men were accounted for except Oswald.