It seems like you're talking the difference between suspicion and proof? Saying "it's obvious they're doing it" when you mean you suspect they're doing it is just going to start an argument.
I was using it in the sense of the parent comment, I.E. if you put $100 in a room unsupervised with a career thief it's "obvious" that he'll walk out with it. Another way of phrasing it would be, "anyone who isn't suspicious appears negligent."