One is a security breach the authorized person never saw happen. The other is a favor the authorized person chose to do. Treating them as the same problem is why sensor technology alone never fully solves either.
In casual conversation, "tailgating" and "piggybacking" get used interchangeably — both describe a second person getting through a secured door on someone else's credential. In physical security engineering, that interchangeability is a mistake, because the two scenarios differ on exactly one variable that determines what actually fixes them: does the authorized person know it's happening?Get that variable wrong and you'll spend a security budget on sensors and interlocks that were never going to solve the problem you actually have.