The chief problem, as Schneier points out, is that many people confuse "suspicious" with "unusual". "Unusual" is an easier word to define: anything that doesn't normally happen within the scope of the life experience of the observer.

Usually, things that are "suspicious" are "unusual". However, things that are unusual aren't inherently suspicious. Being "suspicious" requires the ability to assess risk and threat: a property which people are notoriously bad at, unless specifically trained. And most of us (myself included) aren't trained in that regard.