In season 6 of Supernatural...
Sam is brought back from hell without his soul.
We spend a good bit of time in the early episodes of this season trying to figure out what's wrong. Eventually, Castiel figured it out by performing what is later humorously referred to as an "Angelic Cavity Search" or "Soulonoscopy". However, not much later in the season, creatures of presumably lesser power are easily able to sense the problem even from across the room.
The Alpha Vampire mentions this in "Family Matters" (S6E7) The Leprechaun also calls it out, in "Clap Your Hands If You Believe" (S6E9)
In previous seasons, we've been led to believe that Angels are among the ultimate powers when it comes to supernatural beings. Their power is just a bit below that of pagan deities, super-high-ranking demons, and Arch-Angels (who seem to be just below God).
So, why is it that so many other seemingly lower monsters are able to figure out this problem without such intense investigation as Castiel required?
P.S.: Please note that I haven't seen anything past S6E9 yet. So, if any answers require spoilers from later episodes please make special mention of that and use appropriate Markdown.