Tailgating

Tailgating is following somebody through a door that has just been opened for them.

Every access control has this weakness, because it authenticates one person and then stays open for a few seconds. Holding the door is normal politeness, and refusing feels rude, which is exactly what makes it work. High-visibility clothing, a full box, a lanyard glimpsed at distance and a confident walk are usually enough. The fix is not to make people ruder, it is to design entrances where holding the door is not the same as admitting somebody.