No, I'm not talking about the Browns fans drinking in the parking lot. I'm not even talking about the road-ragers who think that we'll drive differently just because they're close enough to read the fine print on your license plate renewal sticker.
The tailgaters we need to worry about at work are the neatly-dressed people who quietly walk up behind us and expect us to politely hold the door even though they have no right to be in our building. Tailgating is the art of acting like you belong and of using social pressure to convince people to ignore their own rules and policies. Tailgaters practice coming up behind you with just the right balance of professionalism and distractedness so that you believe that they belong. Good tailgaters come prepared with a plausible excuse why you should "be a nice guy" and break the rules – "It's raining ", "My arms are full", "I forgot it in my desk last night", etc. There is no way to identify a scammer just by looking at him or her.
Tailgaters represent a real risk for your organization. Once in the building, they can steal information, compromise systems or worse. If the intruder is a disgruntled claimant, a former employee or a significant other, they could be attempting to get into the building for violent reasons.
Whatever your entry control procedures are, you should have a strict "no tailgating" policy. Do not let staff hold the door for anyone until you are sure that they are authorized to be in the area. If your building uses security badges and someone tries to follow you through a controlled door, demand to see their badge. If they are a visitor, politely escort them to your main entrance and get them properly signed in. (You do have a Visitor's Log, of course. If not, here's a template you can use. Visitor Logs are surprisingly effective at deflecting these criminals to other, easier targets.)
You also need to know what your office's emergency reaction plan is before someone forces their way in. Know who to call and how to report the breach. Don't put yourself in harm's way but do not allow the intruder to wander your halls unchallenged.