Tailgating is slipping through a controlled door behind someone with legitimate access, usually by exploiting politeness: nobody wants to shut a door in a stranger’s face. It is the most common physical security failure in offices, and no badge reader stops it on its own.
Controls are layered: culture (staff are expected to challenge or report), design (speed lanes or interlocking doors where risk justifies them), and records (a visitor process that means everyone inside is badged and logged, so an unbadged person stands out). The badge system only works when tailgating is treated as everyone’s job to prevent.