Truly said in his WC testimony: that he never made a "complete check" of his employees, but merely "saw a group of the employees over there on the floor and I noticed this boy wasn't with them." That's not an official roll call.
FBI statements from 73 TSBD employees revealed that 17 employees never went back inside the building after the shots were fired. So, if 17 of them never went back inside, Oswald wouldn't be the only one missing for a roll call.