Someone came in on another person's open.
The gate opened once; two vehicles came through. You see the clip and decide — instead of finding out from a broken lock.
A code used by the wrong vehicle.
A customer's code, but not a vehicle they've ever arrived in. A lent code, a shared code, or a new car — you decide, with the clip in front of you.
Unusual for your site.
Someone in a row at an hour nobody has been there. A vehicle where vehicles don't go. Rare for here — not rare in general.
No access system? It still learns what's normal at your site and flags what isn't. Everything it flags comes as a question with a clip — it never accuses, and it never dispatches on its own.