Deviations come in two varieties. One is behaviors outside of the norm. If someone normally laughs heartily at a joke but in one case they say “that was funny” but don’t laugh, it’s possible they aren’t being honest.
The other change is normal behaviors the occur out of context. Imagine you notice that an employee can’t make eye contact when they’re nervous. If they do this in a situation where they shouldn’t be nervous you might wonder if there’s something they aren’t saying.